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

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.