.. include:: autodoc_abbr_options_c.rst .. _`sec:psithonInput`: ================================== Psithon: Structuring an Input File ================================== To allow arbitrarily complex computations to be performed, |PSIfour| was built upon the Python interpreter. However, to make the input syntax simpler, some pre-processing of the input file is performed before it is interpreted, resulting in Python syntax that is customized for PSI, termed Psithon. In this section we will describe the essential features of the Psithon language. |PSIfour| is distributed with an extensive test suite, described in section :ref:`apdx:testSuite`; the input files for these test cases can be found in the samples subdirectory of the top-level |PSIfour| source directory, and should serve as useful examples. .. index:: physical constants .. _`sec:physicalConstants`: Physical Constants ================== For convenience, the Python interpreter will execute the contents of the |psirc| file in the current user's home area (if present) before performing any tasks in the input file. This allows frequently used python variables to be automatically defined in all input files. For example, if we repeatedly make use of the universal gravitational constant, the following line could be placed in the |psirc| file :: UGC = 6.67384E-11 # m^3 / kg^-1 s^-2 which would make the variable ``UGC`` available in all |PSIfour| input files. For convenience, the physical constants used within the |PSIfour| code (which are obtained from the 3rd edition of the IUPAC Green book [Cohen:GreenBook:2008]_) are also automatically loaded as Psithon variables (before |psirc| is loaded, so that |psirc| values can be overridden by the user). .. _`table:physconst`: The physical constants used within |PSIfour|, which are automatically made available within all |PSIfour| input files. .. literalinclude:: /../../../../lib/python/physconst.py :lines: 3- The ``psi_`` prefix is to prevent clashes with user-defined variables in |PSIfour| input files. .. index:: molecule; specification .. _`sec:moleculeSpecification`: Molecule and Geometry Specification =================================== |PSIfour| has a very flexible input parser that allows the user to provide geometries as Cartesian coordinates, Z-matrix variables, or a combination of both. The use of fixed values and variables are supported for both. For example, the geometry for H\ :sub:`2` can be specified a number of ways, using the ``molecule`` keyword:: molecule{ H H 1 0.9 } or molecule{ H H 1 r r = 0.9 } or molecule{ H1 H2 H1 0.9 } or molecule{ H 0.0 0.0 0.0 H 0.0 0.0 0.9 } or molecule{ H 0.0 0.0 0.0 H 0.0 0.0 r r = 0.9 } or molecule{ H 0.0 0.0 -r H 0.0 0.0 r r = 0.45 } Blank lines are ignored and, unlike regular Python syntax, indentation within the ``molecule`` block does not matter, although the ``molecule`` keyword itself must be aligned within the input according to standard Python syntax. For more examples of geometry specification, see the :srcsample:`mints1` input file in the samples folder. It is also possible to mix Cartesian and Z-matrix geometry specifications, as demonstrated in the :srcsample:`mints4` and :srcsample:`mints6` sample input files. For example, consider the following geometry specification, taken from the :srcsample:`mints6` input:: molecule alanine { N -1.527107413251 0.745960643462 0.766603000356 C -0.075844098953 0.811790225041 0.711418672248 C 0.503195220163 -0.247849447550 -0.215671574613 O -0.351261319421 -0.748978309671 -1.089590304723 O 1.639498336738 -0.571249748886 -0.174705953194 H -1.207655674855 -0.365913941094 -0.918035522052 # First, remove the H from the alpha carbon. This line could be deleted # and is only included for completeness #H 0.429560656538 0.717651915252 1.673774709694 # Now patch it, using a Z Matrix specification. This patch can be applied # anywhere in the coord specification, as long as it appears lower than # the atoms referenced, as is usual for Z-Matrices C 2 rCC 3 aCCC 1 dCCCN H 7 rCH1 2 aHCC1 3 dHCCC1 H 7 rCH2 2 aHCC2 3 dHCCC2 H 7 rCH3 2 aHCC3 3 dHCCC3 H 0.221781602033 1.772570540211 0.286988509018 H -1.833601608592 0.108401996052 1.481873213172 H -1.925572581453 1.640882152784 0.986471814808 aCCC = 108.0 rCC = 1.4 dCCCN = 120 rCH1 = 1.08 rCH2 = 1.08 rCH3 = 1.08 aHCC1 = 109.0 aHCC2 = 109.0 aHCC3 = 109.0 dHCCC1 = 0.0 dHCCC2 = 120.0 dHCCC3 = 240.0 } Here, we remove the hydrogen from the alpha carbon of glycine and replace it with a methyl group. Applying this patch using Cartesian coordinates is difficult, because it depends on the orientation of the existing glycine unit. In this example, we use Z-Matrix coordinates to define the methyl group, and define the orientation in terms of the existing glycine Cartesian coordinates which is much easier to visualize than the corresponding Cartesian-only approach. .. index:: molecule; multiple in input file .. _`sec:multipleMolecules`: Multiple Molecules ^^^^^^^^^^^^^^^^^^ To facilitate more elaborate computations, it is possible to provide a name for each molecule, and tell |PSIfour| which one should be used in a given calculation. For example, consider the following input file:: molecule h2{ H H 1 0.9 } set basis cc-pvdz set reference rhf energy('scf') molecule h{ H } set basis cc-pvdz set reference uhf energy('scf') Here, two separate jobs are performed on two different molecules; the first is performed on H\ :sub:`2`, while the second is for H atom. The last molecule to be specified is the "active" molecule by default. To explicitly activate a named molecule, the activate keyword is provided. Using this keyword, the above input file can be equivalently written as follows:: molecule h2{ H H 1 0.9 } molecule h{ H } activate(h2) set basis cc-pvdz set reference rhf energy('scf') activate(h) set basis cc-pvdz set reference uhf energy('scf') Note that whenever the molecule is changed, the basis set must be specified again. The following section provides more details about the job control keywords used in the above examples. .. index:: triple: setting; keywords; molecule pair: molecule; charge pair: molecule; multiplicity pair: molecule; symmetry pair: molecule; no_reorient pair: molecule; units .. _`sec:moleculeKeywords`: Molecule Keywords ^^^^^^^^^^^^^^^^^ In addition to specifying the geometry, additional information can be provided in the :samp:`molecule {optional_molecule_name} \\{...\\}` block. If two integers :samp:`{charge} {multiplicity}` are encountered on any line of the molecule block, they are interpreted as the molecular charge and multiplicity (:math:`2 \times M_s + 1`), respectively. The symmetry can be specified by a line reading :samp:`symmetry {symbol}`, where :samp:`{symbol}` is the :ref:`Schönflies symbol ` of the (Abelian) point group to use for the computation. This need not be specified, as the molecular symmetry is automatically detected by |PSIfour|. Certain computations require that the molecule is not reoriented; this can be achieved by adding either ``no_reorient`` or ``noreorient``. By default, |Angstrom| units are used; this is changed by adding a line that reads :samp:`units {spec}`, where :samp:`{spec}` is one of ``ang``, ``angstrom``, ``a.u.``, ``au``, or ``bohr``. .. index:: single: Ghost Atoms single: molecule; ghost .. _`sec:ghosts`: Ghost Atoms ^^^^^^^^^^^ While many common computations, such as SAPT and counterpoise corrections, can be greatly simplified using the notation described in :ref:`sec:fragments`, manual specification of ghost atoms is sometimes required. Either :: molecule he2 { He Gh(He) 1 2.0 } or :: molecule he2 { He @He 1 2.0 } will generate a helium dimer, with the second atom ghosted, *i.e.*, possessing basis functions but no electrons or nuclear charge. See :srcsample:`dfmp2_1` and :srcsample:`ghosts` for a demonstration of both mechanisms for specifying ghost atoms. .. index:: single: PubChem single: molecule; PubChem .. _`sec:pubchem`: Geometries from the `PubChem `_ Database ========================================================================== Obtaining rough starting guess geometries can be burdensome. The Z-matrix coordinate system was designed to provide chemists with an intuitive method for guessing structures in terms of bond lengths and angles. While Z-matrix input is intuitive for small molecules with few degrees of freedom, it quickly becomes laborious as the system size grows. To obtain a reasonable starting guess geometry, |PSIfour| can take a chemical name as input; this is then used to attempt to retrieve Cartesian coordinates from the [PubChem]_ database. For example, to run a computation on benzene, we can use the following molecule specification:: molecule benzene { pubchem:benzene } If the computer is connected to the internet, the above code will instruct |PSIfour| to search PubChem for a starting structure. The search is actually performed for compounds whose name *contains* "benzene", so multiple entries will be returned. If the name provided ("benzene" in the above example) exactly matches one of the results, that entry will be used. If no exact match is found the results, along with a unique chemical identifier (CID), are printed to the output file, prompting the user to provide a more specific name. For example, if we know that we want to run a computation on a compound whose name(s) contain "benzene", but we're not sure of the exact IUPAC name, the following input can be used:: molecule benzene { pubchem:benzene* } Appending the "*" prevents an exact match from being found and, at the time of writing, the following results are displayed in the output file:: Chemical ID IUPAC Name 241 benzene 7371 benzenesulfonic acid 91526 benzenesulfonate 244 phenylmethanol 727 1,2,3,4,5,6-hexachlorocyclohexane 240 benzaldehyde 65723 benzenesulfonohydrazide 74296 N-phenylbenzenesulfonamide 289 benzene-1,2-diol 243 benzoic acid 7370 benzenesulfonamide 636822 1,2,4-trimethoxy-5-[(E)-prop-1-enyl]benzene 7369 benzenesulfonyl chloride 12932 N-[2-di(propan-2-yloxy)phosphinothioylsulfanylethyl]benzenesulfonamide 7505 benzonitrile 78438 N-[anilino(phenyl)phosphoryl]aniline 12581 3-phenylpropanenitrile 517327 sodium benzenesulfonate 637563 1-methoxy-4-[(E)-prop-1-enyl]benzene 252325 [(E)-prop-1-enyl]benzene Note that some of these results do not contain the string "benzene"; these compounds have synonyms containing that text. We can now replace the "benzene*" in the input file with one of the above compounds using either the IUPAC name or the CID provided in the list, *viz*:: molecule benzene { pubchem:637563 } or molecule benzene { pubchem:1-methoxy-4-[(E)-prop-1-enyl]benzene } Some of the structures in the database are quite loosely optimized and do not have the correct symmetry. Before starting the computation, |PSIfour| will check to see if the molecule is close to having each of the possible symmetries, and will adjust the structure accordingly so that the maximum symmetry is utilized. The standard keywords, described in Sec. :ref:`sec:moleculeKeywords`, can be used in conjuction to specify charge, multiplicity, symmetry to use, *etc.* . .. index:: symmetry, Cotton-ordering .. _`sec:symmetry`: Symmetry ======== For efficiency, |PSIfour| can utilize the largest Abelian subgroup of the full point group of the molecule. Concomitantly a number of quantities, such as |globals__socc| and |globals__docc|, are arrays whose entries pertain to irreducible representations (irreps) of the molecular point group. Ordering of irreps follows the convention used in Cotton's :title:`Chemical Applications of Group Theory`, as detailed in Table :ref:`Irreps `. We refer to this convention as "Cotton Ordering" hereafter. .. _`table:irrepOrdering`: .. table:: Ordering of irreducible representations (irreps) used in |PSIfour| +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ | Point Group | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | +================+=============+================+================+================+=============+================+================+================+ | :math:`C_1` | :math:`A` | | | | | | | | +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ | :math:`C_i` | :math:`A_g` | :math:`A_u` | | | | | | | +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ | :math:`C_2` | :math:`A` | :math:`B` | | | | | | | +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ | :math:`C_s` | :math:`A'` | :math:`A''` | | | | | | | +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ | :math:`D_2` | :math:`A` | :math:`B_1` | :math:`B_2` | :math:`B_3` | | | | | +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ | :math:`C_{2v}` | :math:`A_1` | :math:`A_2` | :math:`B_1` | :math:`B_2` | | | | | +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ | :math:`C_{2h}` | :math:`A_g` | :math:`B_g` | :math:`A_u` | :math:`B_u` | | | | | +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ | :math:`D_{2h}` | :math:`A_g` | :math:`B_{1g}` | :math:`B_{2g}` | :math:`B_{3g}` | :math:`A_u` | :math:`B_{1u}` | :math:`B_{2u}` | :math:`B_{3u}` | +----------------+-------------+----------------+----------------+----------------+-------------+----------------+----------------+----------------+ For example, water (:math:`C_{2v}` symmetry) has 3 doubly occupied :math:`A_1` orbitals, as well as 1 each of :math:`B_1` and :math:`B_2` symmetry; the corresponding |globals__docc| array is therefore:: DOCC = [3, 0, 1, 1] Although |PSIfour| will detect the symmetry automatically, and use the largest possible Abelian subgroup, the user might want to run in a lower point group. To do this the ``symmetry`` keyword can be used when inputting the molecule (see Sec. :ref:`sec:moleculeSpecification`). In most cases the standard Schönflies symbol (one of ``c1``, ``c2``, ``ci``, ``cs``, ``d2``, ``c2h``, ``c2v``, ``d2h`` will suffice. For certain computations, the user might want to specify which particular subgroup is to be used by appending a unique axis specifier. For example when running a computation on a molecule with :math:`D_{2h}` symmetry in :math:`C_{2v}`, the :math:`C_2` axis can be chosen as either the :math:`x`, the :math:`y`, or the :math:`z`; these can be specified by requesing the symmetry as ``c2vx``, ``c2vy``, or ``c2vz``, respectively. Likewise the ``c2x``, ``c2y``, ``c2z``, ``c2hx``, ``c2hy``, and ``c2hz`` labels are valid. For :math:`C_s` symmetry the labels ``csx``, ``csy``, and ``csz`` request the :math:`yz`, :math:`xz`, and :math:`xy` planes be used as the mirror plane, respectively. If no unique axis is specified, |PSIfour| will choose an appropriate subgroup. Certain types of finite difference computations, such as numerical vibrational frequencies, might lower the symmetry of the molecule. When this happens symmetry-dependent arrays, such as |globals__socc|, are automatically remapped to the lower symmetry. For example, if we were to investigate the :math:`^2B_1` state of water cation, we can specify SOCC = [0, 0, 1, 0] in the input file. If any ensuing computations lower the symmetry, the above array will be appropriately remapped. For example, reducing the symmetry to :math:`C_s` (with the molecular plane defining the mirror plane), the above array will be automatically interpreted as: SOCC = [0, 1] Some caution is required, however. The :math:`^2A_1` state can be obtained with the SOCC = [1, 0, 0, 0] specification, which would become SOCC = [1, 0] under the above-mentioned reduction in symmetry. The :math:`^2B_2` state, whose singly-occupied orbitals are SOCC = [0, 0, 0, 1] would be mapped to SOCC = [1, 0] which is the same occupation as the :math:`^2A_1` state. In this case, the :math:`^2A_1` state is lower in energy, and is not problematic. The distorted geometries for the :math:`^2B_2` state are excited states that are subject to variational collapse. One way to obtain reliable energies for these states is to use a multi-state method; in this case it's easier to run the entire computation in the lowest symmetry needed during the finite difference procedure. .. index:: molecule; multiple fragments .. _`sec:fragments`: Non-Covalently Bonded Molecule Fragments ======================================== |PSIfour| has an extensive range of tools for treating non-covalent intermolecular forces, including counterpoise corrections and symmetry adapted perturbation theory methods. These require the definition of which fragments are interacting within the complex. |PSIfour| provides a very simple mechanism for doing so; simply define the complex's geometry using the standard Cartesian, Z-matrix, or mixture thereof, specifications and then place two dashes between nonbonded fragements. For example, to study the interaction energy of ethane and ethyne molecules, we can use the following molecule block:: molecule eneyne { 0 1 C 0.000000 -0.667578 -2.124659 C 0.000000 0.667578 -2.124659 H 0.923621 -1.232253 -2.126185 H -0.923621 -1.232253 -2.126185 H -0.923621 1.232253 -2.126185 H 0.923621 1.232253 -2.126185 -- 0 1 C 0.000000 0.000000 2.900503 C 0.000000 0.000000 1.693240 H 0.000000 0.000000 0.627352 H 0.000000 0.000000 3.963929 } In this case, the charge and multiplicity of each interacting fragment is explicitly specified. If the charge and multiplicity are specified for the first fragment, it is assumed to be the same for all fragments. When considering interacting fragments, the overall charge is simply the sum of all fragment charges, and any unpaired electrons are assumed to be coupled to yield the highest possible :math:`M_s` value. Having defined a molecule containing fragments like ``eneyne`` above, it is a simple matter to perform calculations on only a subset of the fragments. For instance, the commands below run a scf first on the ethene fragment alone (``extract_subsets(1)`` pulls out fragment 1 as Real atoms and discards remaining fragments) and next on the ethene fragment with the ethyne fragment ghosted (``extract_subsets(1,2)`` pulls out fragment 1 as Real atoms and sets fragment 2 as Ghost atoms). For beyond bimolecular complexes, arrays can be used, e.g. ``extract_subsets(2,[1,3])``:: mA = eneyne.extract_subsets(1) energy('scf') clean() mAcp = eneyne.extract_subsets(1,2) energy('scf') .. index:: single: basis set; specification triple: setting; keywords; C-side .. _`sec:jobControl`: Job Control =========== |PSIfour| comprises a number of modules, written in C++, that each perform specific tasks and are callable directly from the Python front end. Each module recognizes specific keywords in the input file, detailed in Appendix :ref:`apdx:options_c_module`, which control its function. The keywords can be made global, or scoped to apply to certain specific modules. The following examples demonstrate some of the ways that global keywords can be specified:: # all equivalent set globals basis cc-pVDZ set basis cc-pVDZ set globals basis = cc-pVDZ set basis = cc-pVDZ set globals{ basis cc-pVDZ } set { basis cc-pVDZ } set { basis = cc-pVDZ } Note the lack of quotes around ``cc-pVDZ``, even though it is a string. The Psithon preprocessor automatically wraps any string values in ``set`` commands in strings. The last three examples provide a more convenient way for specifying multiple keywords:: set { basis = cc-pVDZ print = 1 reference = rhf } For arguments that require an array input, standard Python list syntax should be used, *viz.*:: set { docc = [3, 0, 1, 1] } List/matrix inputs may span multiple lines, as long as the opening ``[`` is on the same line as the name of the keyword. Any of the above keyword specifications can be scoped to individual modules, by adding the name of the module after the ``set`` keyword. Omitting the module name, or using the name ``global`` or ``globals`` will result in the keyword being applied to all modules. For example, in the following input :: molecule{ o h 1 roh h 1 roh 2 ahoh roh = 0.957 ahoh = 104.5 } set basis cc-pVDZ set ccenergy print 3 set scf print 1 energy('ccsd') the basis set is set to cc-pVDZ throughout, the SCF code will have a print level of 1 and the ccenergy code, which performs coupled cluster computations, will use a print level of 3. In this example a full CCSD computation is performed by running the SCF code first, then the coupled cluster modules; the ``energy()`` Python helper function ensures that this is performed correctly. Note that the Python interpreter executes commands in the order they appear in the input file, so if the last four commands in the above example were to read :: set basis cc-pVDZ energy('ccsd') set ccenergy print 3 set scf print 1 the commands that set the print level would be ineffective, as they would be processed after the CCSD computation completes. .. index:: basis set; multiple within molecule .. _`sec:psithonBasissets`: Assigning Basis Sets ==================== While the above syntax will suffice for specifying basis sets in most cases, the user may need to assign basis sets to specific atoms. To achieve this, a ``basis`` block can be used. We use a snippet from the :srcsample:`mints2` sample input file, which performs a benzene SCF computation, to demonstrate this feature. :: basis { assign DZ assign C 3-21G assign H1 sto-3g assign C1 sto-3g } The first line in this block assigns the DZ basis set to all atoms. The next line then assigns 3-21G to all carbon atoms, leaving the hydrogens with the DZ basis set. On the third line, the hydrogen atoms which have been specifically labelled as ``H1`` are given the STO-3G basis set, leaving the unlabelled hydrogen atoms with the DZ basis set. Likewise, the fourth line assigns the STO-3G basis set to just the carbon atoms labelled ``C1``. This bizzare example was constructed to demonstrate the syntax, but the flexibility of the basis set specification is advantageous, for example, when selectivily omitting diffuse functions to make computations more tractable. .. index:: basis set; auxiliary In the above example the basis sets have been assigned asymmetrically, reducing the effective symmetry from :math:`D_{6h}` to :math:`C_{2v}`; |PSIfour| will detect this automatically and run in the appropriate point group. The same syntax can be used to specify basis sets other than that used to define orbitals. For example, :: set df_basis_mp2 cc-pvdz-ri or basis { assign cc-pVDZ-RI df_basis_mp2 } are both equivalent ways to set the auxiliary basis set for density fitted MP2 computations. To assign the aug-cc-pVDZ-RI to carbon atoms, the following command is used:: basis { assign C aug-cc-pVDZ-RI df_basis_mp2 } When most popular basis sets are being used, including Dunning and Pople-style, the SCF, DF-MP2, and SAPT codes will chose the appropriate auxiliary basis set automatically according to :ref:`apdx:basisFamily`, unless instructed otherwise by setting the auxiliary basis set in the input. Finally, we note that the ``basis {...}`` block may also be used for defining basis sets, as detailed in Sec. :ref:`sec:basisUserDefined`. .. index:: memory .. _`sec:memory`: Memory Specification ==================== By default, |PSIfour| assumes that 256 Mb of memory are available. While this is enough for many computations, many of the algorithms will perform better if more is available. To specify memory, the ``memory`` keyword should be used. The following lines are all equivalent methods for specifying that 2 Gb of RAM is available to |PSIfour|:: # all equivalent memory 2 Gb memory 2000 Mb memory 2000000 Kb One convenient way to override the |PSIfour| default memory is to place a memory command in the |psirc| file (Sec. :ref:`sec:psirc`). For example, the following makes the default memory 2 Gb. :: set_memory(2000000000) However, unless you're assured of having only one job running on a node at a time (and all nodes on the filesystem with |psirc| have similar memory capacities), it is advised to set memory in the input file on a per-calculation basis. .. note:: For parallel jobs, the ``memory`` keyword represents the total memory available to the job, *not* the memory per thread. .. _`sec:psiVariables`: Return Values and PSI Variables =============================== To harness the power of Python, |PSIfour| makes the most pertinent results of each computation are made available to the Python interpreter for post-processing. To demonstrate, we can embellish the previous example of H\ :sub:`2` and H atom:: molecule h2{ H H 1 0.9 } set basis cc-pvdz set reference rhf h2_energy = energy('scf') molecule h{ H } set basis cc-pvdz set reference uhf h_energy = energy('scf') D_e = psi_hartree2kcalmol*(2*h_energy - h2_energy) print"De=%f"%D_e The :py:func:`~driver.energy` function returns the final result of the computation, the requested total energy in Hartrees, which we assign to a Python variable. The two energies are then converted to a dissociation energy and printed to the output file using standard Python notation. Generally, there are multiple quantities of interest. Appendix :ref:`apdx:psivariables_module` lists PSI variables variables set by each module. These can be accessed through the ``get_variable()`` function. For example, after performing a density fitted MP2 computation, both the spin component scaled energy and the unscaled MP2 energy are made available:: e_mp2=get_variable('MP2 TOTAL ENERGY') e_scs_mp2=get_variable('SCS-MP2 TOTAL ENERGY') Each module and the Python driver set PSI variables over the course of a calculation. The values for all can be printed in the output file with the input file command ``print_variables()``. Note that PSI variables accumulate over a |PSIfour| instance and are not cleared by ``clean()``. So if you run in a single input file a STO-3G FCI followed by a aug-cc-pVQZ SCF followed by a ``print_variables()`` command, the last will include both :psivar:`SCF TOTAL ENERGY ` and :psivar:`FCI TOTAL ENERGY `. Don't get excited that you got a high-quality calculation cheaply. .. _`sec:loops`: Loops ===== Python provides many control structures, which can be used within |PSIfour| input files. For example, to loop over three basis sets, the following code can be used:: basis_sets=["cc-pVDZ","cc-pVTZ","cc-pVQZ"] for basis_set in basis_sets: set basis = $basis_set energy('scf') The declaration of ``basis_set`` is completely standard Python, as is the next line, which iterates over the list. However, because the Psithon preprocessor wraps strings in quotes by default, we have to tell it that ``basis_set`` is a Python variable, not a string, by prefixing it with a dollar sign. The geometry specification supports delayed initialization of variable, which permits potential energy scans. As an example, we can scan both the angle and bond length in water:: molecule h2o{ O H1 R H1 R2 A } Rvals=[0.9,1.0,1.1] Avals=range(102,106,2) set basis cc-pvdz set scf e_convergence=11 for R in Rvals: h2o.R = R for A in Avals: h2o.A = A energy('scf') The declarations of ``Rvals`` and ``Avals`` are both completely standard Python syntax. Having named our molecule ``h2o`` we can then set the values of ``R`` and ``A`` within the loops. Note that we do not need the dollar sign to access the Python variable in this example; that is required only when using Python variables with the ``set`` keyword. .. _`sec:resultsTables`: Tables of Results ================= The results of computations can be compactly tabulated with the :py:func:`~text.Table` Psithon function. For example, in the following potential energy surface scan for water :: molecule h2o { O H 1 R H 1 R 2 A } Rvals=[0.9,1.0,1.1] Avals=range(100,102,2) table=Table(rows=["R","A"], cols=["E(SCF)","E(SCS)","E(DFMP2)"]) set basis cc-pvdz for R in Rvals: h2o.R = R for A in Avals: h2o.A = A energy('df-mp2') escf = get_variable('SCF TOTAL ENERGY') edfmp2 = get_variable('DF-MP2 TOTAL ENERGY') escsmp2 = get_variable('SCS-DF-MP2 TOTAL ENERGY') table[R][A] = [escf, escsmp2, edfmp2] print table relative=table.copy() relative.absolute_to_relative() print relative we first define a table (on line 10) with two row indices and three column indices. As the potential energy scan is performed, the results are stored (line 22) and the final table is printed to the output file (line 24). The table is converted from absolute energies to relative energies (in |kcalpermol|) on line 26, before being printed again. The relative energies are reported with respect to the lowest value in each column. More examples of how to control the formatting of the tables can be found in the sample input files provided; see Appendix :ref:`apdx:testSuite` for a complete listing. .. _`sec:wrappers`: Python Wrappers =============== The Python foundations of the |PSIfour| driver and Psithon syntax permit many commonly performed post-processing procedures to be integrated into the |PSIfour| suite. Among these are automated computations of interaction energies through :py:func:`~wrappers.cp`, of a model chemistry applied to a database of systems through :py:func:`~wrappers.database`, and of several model chemistries together approximating greater accuracy through :py:func:`~wrappers.complete_basis_set`. These are discussed separately in section :ref:`sec:psithonFunc`. Note that the options documented for Python functions are placed as arguments in the command that calls the function, not in the ``set globals`` block or with any other ``set`` command.