*Code author: Francesco A. Evangelista and Andrew C. Simmonett*

*Section author: Alexander E. Vaughn*

*Module:* *Keywords*, *PSI Variables*, PSIMRCC

State-specific Multireference coupled cluster theories provide highly accurate energies and properties of electronic states that require a multiconfigurational zeroth-order wavefunction. The PSIMRCC module contained in PSI4 implements the state-specific multireference coupled-cluster approach of Mukherjee and co-workers (Mk-MRCC). This method is implemented and shown to be a powerful tool in [Evangelista:2006:154113] and [Evangelista:2008:124104]. Mk-MRCC is based on the Jeziorski-Monkhorst ansatz [Jeziorski:1981:1668] for the wavefunction,

where are the reference determinants, are reference-specific excitation operators, and are expansion coefficients obtained through diagonalization of the Mk-MRCC effective Hamiltonian matrix that allows the various reference determinants to interact. As an example of how this works the Mk-MRCCSD excitation operators for each reference is contracted two-body terms

where

and

The Mk-MRCC energy is a chosen eigenvalue of the effective Hamiltonian,

where

PSI4 currently implements Mk-MRCC with singles and doubles [Mk-MRCCSD] and Mk-MRCCSD with perturbative triples [Mk-MRCCSD(T)] as formulated in [Evangelista:2010:074107]. A companion perturbation method (Mk-MRPT2) has been developed based on the Mukherjee formalisim as shown in [Evangelista:2009:4728].

The current version of the code is limited to reference active spaces in which all determinants are connected to each other by no more than two excitations. In practice, this usually means that the active space can have at most two particles, or at most two holes. Examples would include CAS(2,2), CAS(2,8), CAS(4,3), etc., where CAS(n,m) refers to a complete-active-space configuration interaction (CAS-CI) reference with n electrons in m orbitals. If the user specifies active spaces that do not fit these limitations, then the code will still run, but some relevant determinants will be missing, and the answer obtained will be an approximation to the true Mk-MRCC procedure.

The PSIMRCC code itself does not perform orbital optimization.
Hence, the references used might be considered CAS-CI references,
but not CASSCF references (CASSCF implies that the orbitals have been
optimized specifically to minimize the energy of the CAS-CI reference).
However, if one wishes to use two-configuration self-consistent-field
(TCSCF) orbitals, those can be obtained using the multi-configuration
self-consistent-field (MCSCF) component of PSIMRCC (specifying
*REFERENCE* to be `twocon`).

This is suitable for describing diradicals. Otherwise, one may use
RHF or ROHF orbitals as input to PSIMRCC. Due to a current limitation
in the code, one must obtain orbitals using PSIMRCC’s MCSCF module
regardless of what orbital type is chosen, `twocon`, `rhf`, or
`rohf`. An example of the MCSCF input is given below.

PSIMRCC is most commonly used for low-spin cases (singlets or open-shell singlets). It is capable of performing computations on higher spin states (e.g., triplets), but in general, not all the required matrix elements have been coded for high-spin cases, meaning that results will correspond to an approximate Mk-MRCC computation for high-spin cases.

The *CORR_WFN* allows you to select one of three methods Mk-MRPT2 [`PT2`], Mk-MRCCSD [`CCSD`], or Mk-MRCCSD(T) [`CCSD_T`]. The *CORR_MULTP* option allows you to select the Slater determinants with a particular value. The *WFN_SYM* keyword is neccesary if you do not want to compute the energy of the all-symmetric state. The *FOLLOW_ROOT* option may be used to follow different roots of the effective Hamiltonian. A value of 1 instructs PSIMRCC to follow the solution with the lowest energy given a certain set of determinants.

```
molecule o2 {
0 3
O
O 1 2.265122720724
units au
}
set {
basis cc-pvtz
}
set mcscf {
reference rohf
docc [3,0,0,0,0,2,1,1] # Doubly occupied MOs
socc [0,0,1,1,0,0,0,0] # Singly occupied MOs
}
set psimrcc {
corr_wfn ccsd # Do Mk-MRCCSD
frozen_docc [1,0,0,0,0,1,0,0] # Frozen MOs
restricted_docc [2,0,0,0,0,1,1,1] # Doubly occupied MOs
active [0,0,1,1,0,0,0,0] # Active MOs
frozen_uocc [0,0,0,0,0,0,0,0] # Frozen virtual MOs
corr_multp 1 # Select the Ms = 0 component
root 1
wfn_sym B1g # Select the B1g state
}
energy('psimrcc')
```

Note that the oxygen molecule has 16 electrons (including core), while
the `docc` array contains only 7 doubly-occupied orbitals (or 14
electrons). Hence, two more electrons are available to place into
the active space (given by `active`), which consists of 2 orbitals.
Thus there are two active electrons in two orbitals. In this particular
example, we are using standard ROHF orbitals for the Mk-MRCCSD procedure,
rather than TCSCF orbitals. Nevertheless, with the present code,
these orbitals must be provided through the MCSCF module, as specified in
the
`set mcscf` section above.

The reference determinants are specified in PSIMRCC via occupational numbers. PSIMRCC requires that four arrays be specified for this purpose.

- Frozen doubly occupied orbitals (
*FROZEN_DOCC*) are doubly occupied in each reference determinant and are not correlated in the MRCC procedure. - Doubly occupied orbitals (
*RESTRICTED_DOCC*) are doubly occupied in each reference determinant and are correlated in the MRCC procedure. - Active orbitals (
*ACTIVE*) are partially occupied in each reference determinant. - Frozen virtual orbitals (
*FROZEN_UOCC*) are unoccupied in all reference determinants and are excluded from the correlated wave function.

The model space is selected by considering all possible occupations of the electrons among the orbitals in the active space that result in determinants with the correct symmetry (*WFN_SYM*) and the correct value specified by the keyword *CORR_MULTP*. Note that this does not consider the multiplicity of the wavefunction. Thus, in order to obtain the wavefunction with a set of reference determinants for an open-shell system you should request a *CORR_MULTP* of 1 within the PSIMRCC module, and select the root of the effective Hamiltonian that corresponds to the state of interest. In addition, the *WFN_SYM* keyword needs to be specified otherwise the wavefunction belonging to the all-symmetric irrep will be selected. In addition, it should be noted that for an open-shell singlet based on two determinants the eigenvector is [], which corresponds to a wavefunction of the following form:

See Appendix *PSIMRCC* for a complete list of PSIMRCC options.