Interface to programs through FCHK files — fchk()

Code author: Andrew C. Simmonett

Section author: Andrew C. Simmonett

Many post-processing tools can read information from Gaussian’s formatted checkpoint (FCHK) files. To allow interoperability with such tools, PSI4 includes a utility to generate FCHK files. Wavefunction information, such as orbitals, densities, orbital energies and basis set information is currently supported, but geometry optimization and vibrational frequency information are not supported at this time. To generate a FCHK file, simply store the wavefunction from the energy calculation, and hand it to the fchk driver function:

energy, wfn = energy('scf', return_wfn=True)

The file will be written to the name passed to the fchk function. Note that for MP2 and CCSD methods, the energy can be computed without the expensive steps required to compute the density, so energy calls for these methods will return a wavefunction that has the Hartree–Fock density. If a density is required for these methods, the user should instead request a gradient computation, to ensure that the density is updated appropriately:

grad, wfn = gradient('mp2', return_wfn=True)
psi4.fchk(wfn, filename, *, debug=False, strict_label=True)[source]

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

Added 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

  • 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)