The analytic gradient with a reduced molecular orbital space for the equation-of-motion coupled-cluster theory: Systematic study of the magnitudes and trends in simple molecules
Kk. Baeck et Si. Jeon, The analytic gradient with a reduced molecular orbital space for the equation-of-motion coupled-cluster theory: Systematic study of the magnitudes and trends in simple molecules, B KOR CHEM, 21(7), 2000, pp. 720-726
The analytic gradient method for the equation-of-motion coupled-cluster sin
gles and doubles (EOM-CCSD) energy has been extended to employ a reduced mo
lecular orbital (MO) space. Not only the innermost core MOs but also some o
f the outermost virtual MOs can be dropped in the reduced MO space, and a s
ubstantial amount of computation time can be reduced without deteriorating
the results, In order to study the magnitudes and trends of the effects of
the dropped MOs, the geometries and vibrational properties of the ground an
d excited states of BF, CO, CN, N-2, AlCl, SiS, P-2, BCl, AlF, CS, SiO, PN
and GeSe are calculated with different sizes of molecular orbital space. Th
e 6-31G* and the aug-cc-pVTZ basis sets are employed for all molecules exce
pt GeSe for which the 6-311G* and the TZV+f sets are used. It is shown that
the magnitudes of the drop-MO effects are about 0.005 Angstrom in bond len
gths and about 1% on harmonic frequencies and IR intensities provided that
the dropped MOs correspond to (1s), (1s,2s,2p), and (1s,2s,2p,3s,3p) atomic
orbitals of the first, the second, and the third row atoms, respectively.
The geometries and vibrational properties of the first and the second excit
ed states of HCN and HNC are calculated by using a drastically reduced virt
ual MO space as well as with the well defined frozen core MO space. The res
ults suggest the possibility of using a very small MO space for qualitative
study of valence excited states.