Natural transition orbitals for the calculation of correlation and excitation energies

We propose a scheme to reduce the virtual space in the second-order approximate coupled-cluster singles-and-doubles (CC2) method, allowing for an accurate description of both ground-state correlation and excitation energies. A set of natural virtual orbitals is constructed using the ground-state MP2 density as well as the configuration-interaction-singles (CIS) excitation vectors. The results show that approximately half of the virtual space can be removed while an accuracy of about 90% for conventional correlation energies is obtained. CC2 excitation energies show in most cases an error of about 2% or less when the virtual space is reduced to half. (C) 2017 Elsevier B.V. All rights reserved.