CC calculations on A1Pi state of AlH

Dear MRCC developers,

I am trying to compute CCSD and CCSDT energies for the A1Pi excited electronic state of the diatomic AlH, but I am having difficulties.

In my input for a CCSD calculation contains the following (I have obtained the Al basis set from Basis Set Exchange):

basis=atomtype
Al:aug-cc-pwCVDZ
H:aug-cc-pVDZ
calc=CCSD
mem=10GB
core=0
scftype=uhf
scfmaxit=500
ccmaxit=200
ccprog=mrcc
cmpgrp=C2v
symm=B1
mult=1
occ=4,2,1,0/5,1,1,0

geom=xyz
2

Al 0 0 0
H 0 0 1.6552

However, this fails, giving the error "Invalid spatial symmetry!". I have also tried including "nstate=2" and/or "nsing=2" in the input. As this is the lowest energy singlet state of this symmetry I would have thought it shouldn't be too difficult to locate. Any ideas as to what I'm doing wrong? Have I missed something important from my input?

Thanks a lot,

Ben

Dear Ben,
The occupation you specified seems to be an open-shell singlet. Please note that CC calculations are not possible with open-shell singlet reference.

Dear Mihaly,

Thank you very much for your quick response. Would multi-reference CC work for calculations on this open shell singlet state?

Thanks again,

Ben

Dear Ben,
In principle, it works, but there may be problems due to the ambiguity related to the selection of the Fermi vacuum (see JCP 117, 980). Another solution is to use some closed-shell reference determinant and perform an EOM-CC calculation for the desired state.