psi4.driver.fchk(wfn, filename, *, debug=False, strict_label=True)[source]

Function to write wavefunction information in wfn to filename in Gaussian FCHK format.

New in version 0.6.



  • wfn (Wavefunction) – set of molecule, basis, orbitals from which to generate fchk file

  • filename (str) – destination file name for FCHK file

  • debug (bool) – returns a dictionary to aid with debugging

  • strict_label (bool) – If true set a density label compliant with what Gaussian would write. A warning will be printed if this is not possible. Otherwise set the density label according to the method name.


  • A description of the FCHK format is http://wild.life.nctu.edu.tw/~jsyu/compchem/g09/g09ur/f_formchk.htm

  • The allowed headers for methods are general and limited, i.e., “Total SCF|MP2|CI|CC Density”, PSI4 will try to find the right one for the current calculation. If strict_label=False the PSI4 method name will be used as label.

  • Not all theory modules in PSI4 are compatible with the FCHK writer. A warning will be printed if a theory module is not supported.

  • Caution! For orbital-optimized correlated methods (e.g. DCT, OMP2) the ‘Orbital Energy’ field contains ambiguous data.


>>> # [1] FCHK file for DFT calculation
>>> E, wfn = energy('b3lyp', return_wfn=True)
>>> fchk(wfn, 'mycalc.fchk')
>>> # [2] FCHK file for correlated densities
>>> E, wfn = gradient('ccsd', return_wfn=True)
>>> fchk(wfn, 'mycalc.fchk')
>>> # [2] Write FCHK file with non-standard label.
>>> E, wfn = gradient('mp2.5', return_wfn=True)
>>> fchk(wfn, 'mycalc.fchk', strict_label=False)