The structures of the electronic states of HCOO. were optimized at CAS
SCF and CASPT2 levels of theory with generally contracted basis sets o
f atomic natural orbital type: [8s4p]/[3s2p] for hydrogen and [14s9p4d
]/[4s3p2d] for carbon and oxygen atoms. Vibrational frequencies were c
alculated to verify the characteristics for some of the optimized stat
ionary points. MRCI calculations were carried out for the a radicals a
t the CASPT2 optimized geometries to provide a test on the higher-orde
r correlation effects. At the CASPT2, level, the lowest electronic sta
te of HCOO. is the (2)A(1) state. The electronic energy of the B-2(2)
state is 6 kJ/mol above. The broken symmetry (2)A' state is predicted
to lie 25 kJ/mol above the (2)A(1) State and to be a transition struct
ure for rearrangement to HOCO.. Single-point MRCI calculations field t
he reverse order for the C-2v symmetric states, (2)A(1) state being pr
edicted to be 9.5 kJ/mol above the B-2(2) state. The (2)A' state is ne
arly degenerate with the (2)A(1) state. Relative energies predicted at
the (unoptimized) MRCI level are almost identical to those found by s
ingle-reference CI calculations.