The frozen virtual natural orbital (FVNO) approach, a very useful reduced scaling technique for calculating ground state correlation energies, is extended to calculate chiroptical properties using coupled cluster response theory. Compared to the correlation energies, properties like dynamic polarizabilities and optical rotation are found to be much more sensitive to the truncation of virtual space in the natural orbital basis, with the errors increasing linearly with respect to the number of frozen virtual natural orbitals. It is observed that the diffuse orbitals which are very important for an accurate description of these properties are truncated first in this approach as these orbitals have very low occupation numbers. Applying this method only for the the non-diffuse virtual space and retaining the diffuse virtual space up to a given cutoff in the correlated calculations yields minimal errors. Based on these observations, a new strategy called FVNO++ is devised, where a second-order perturbed reduced density matrix is diagonalized to obtain perturbed natural orbitals. The truncation of virtual orbitals in this perturbed basis results in very small errors compared to the regular FVNO basis. Also, this formalism when applied in conjunction with local pair natural orbital method (L-PNO++) gives very low errors for optical rotations than the conventional L-PNO method, which takes us a step closer towards the development of an efficient reduced-scaling method for coupled cluster chiroptical properties.

FVNO++: Towards a reduced-scaling method for coupled cluster chiroptical properties

255th National Meeting (2018)

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