I am looking for a way to read .fchk file in psi4, such that I can restart a SCF calculation from such a file. I need this because I am playing with the MO coefficients with an external code, to generate localized orbitals. Then, I would like to check that the SCF calculation indeed converges in only one iteration using psi4. It should be the case because my localization scheme is similar to the Pipek-Mezey one, and implies only (unitary) rotations of pair of orbitals.

I’ve seen on this forum that I am not the only one interested by such a feature, and I am not sure if such option has been provided or not yet.

So my question is simple: is there a way to restart a psi4 calculation using an external .fchk file, and if not is it something that will be provided on a future release ?

If one is still interested by this feature, look at the way psi4 writes a FCHK file here (with renormalization factors and reordering).

So if you read a FCHK file, then you should create the back transformation matrices by simply taking 1/pf factors instead of pf and the transpose.

To read the coefficients from a FCHK file, do as follow:

def read_real_list(text,f):
for line in f:
if re.search(text, line):
n=int(line.rsplit(None, 1)[-1])
var=[]
for i in range((n-1)//5+1):
line = next(f)
for j in line.split():
var += [float(j)]
return var
with open("your_fchk_file.fchk","r") as f:
alpha_coeff = read_real_list("Alpha MO coefficients", f)
f.close()
alpha_MO_coeff = np.zeros((scf_wfn.Ca().np.shape[0],scf_wfn.Ca().np.shape[1]),dtype=float)
ij = 0
for i in range(scf_wfn.Ca().np.shape[1]):
for j in range(scf_wfn.Ca().np.shape[0]):
alpha_MO_coeff[j,i] = alpha_coeff[ij]
ij += 1
# then apply the back transformation from FCHK to PSI4 ordering with prefactors.
# Only when this is done you can fill the coefficients back to PSI4:
scf_wfn.Ca().np[:] = alpha_MO_coeff

If the script works for you then good!
The reordering is left out I assume?

I doubt that will be pursued. FCHK is not a good format to exchange data between programs. Not open, limited in scope and subject to change by Gaussian how they please.

What do you mean by “the reordering is left out” ? The reordering should be accounted by the back transformation before setting the MO coefficients in the psi4 wavefunction object. I didn’t write the back transformation in this script so maybe that’s confusing, but this back transformation can be simply constructed by looking at the way psi4 writes the FCHK.

I agree with you for the FCHK format. I have kept this feature for my code to have an interface with Gaussian, but I have now built things differently for psi4 and other codes