Optimized virtual orbitals for correlated calculations:: Towards large scale CCSD(T) calculations of molecular dipole moments and polarizabilities

The optimized virtual orbitals space (OVOS) approach is applied to coupled cluster CCSD(T) calculations of dipole moments and dipole polarizabilities of selected systems. Particular attention is paid to OVOS reduced to about 50% of the full virtual space, which leads to the computer time savings larger than one order of magnitude. The fluorine anion, the carbon monoxide, formaldehyde, thiophene, and a push-pull I amino-4-nitrobutadiene molecules serve as testing examples. For all systems are OVOS results compared with the full virtual space CCSD(T) results. Augmented correlation consistent aug-cc-pVXZ basis sets (X=D, T, and also Q, 5 in some applications) were used in a systematic way. It is shown that the performance of OVOS improves with increasing the basis set size. OVOS method is shown to be useful in reliable extrapolations to the basis set limit. With the standard aug-cc-pVTZ basis set OVOS CCSD(T) dipole moments differ from the full virtual space results typically by 0.002 a.u. The largest difference is 0.006 a.u. Components of the dipole polarizabilities differ typically by 0.02-0.05 a.u. For the F- anion larger basis sets are needed. The difference between the OVOS and the full space CCSD(T) polarizability with the daug-cc-pV5Z basis set is 0.32 a.u. (2.0% of the total value); the complete basis set limits differ by 0.07 a.u. (c) 2006 Elsevier B.V. All rights reserved.