Pseudonatural Orbitals as a Basis for the Superposition of Configurations. I. He2+

Abstract

The use of pseudonatural orbitals (PNO) is proposed to improve the rate of convergence in the superposition of configurations (SOC). Natural orbitals are determined for selected electron pairs in the Hartree—Fock field of the n−2 electron core and are then used as the basis for the total SOC calculation. Since these natural orbitals are not natural for the n-electron system they are considered false or pseudonatural orbitals when used in the n-electron problem. The PNO basis has been applied to He2+ and H3 to test the convergence. Complete results are reported here only for He2+. The PNO's are quite successful in speeding up the convergence of the SOC and rendering the calculation of correlation energy quite practical in general. Gaussian-type orbitals (GTO) are used throughout and were not a serious impediment to obtaining quantitative accuracy. In fact the large number of unoccupied Hartree—Fock orbitals consequent upon the use of a GTO basis permit a straightforward determination of the PNO orbitals.

Frozen natural orbital coupled-cluster theory: Forces and application to decomposition of nitroethane

“…[15][16][17][18][19][20][21][22] Perhaps the most powerful method of doing so is to use frozen natural orbitals ͑FNOs͒. 18,20,[23][24][25][26][27][28] These orbitals use information from an approximate one-particle reduced density matrix to choose the best subset of one-particle orbitals within which to perform a correlated calculation. When using FNOs based on the MBPT͑2͒ density matrix, 29,30 this truncated orbital set has been shown to be surprisingly effective at truncating larger basis sets, allowing ϳ50% of a modified unoccupied orbital set to be removed without significant changes to ground state CC energies and density matrices.…”

Compressed representation of dispersion interactions and long-range electronic correlations

“…[26][27][28] NOs, often extracted from affordable MP2 computations, are also extremely useful in local correlation methods 29,30 , as exemplified by Pair Natural Orbitals (PNOs), 31,32 that tailor the virtual space to each electron pair, and that are extracted from the appropriate density matrix by SVD. PNOs were recently revisited with much success for accurate local correlation methods.…”

Interaction energy surfaces for Li(22S) and Li (22P) with H2

“…The negative ion H ; state formed here is also ~elated to the lowest energy resonance state of H2 considered responsible for enhanced vibrational excitation at low incide nt electron energies [3]. Three electrons are now interpenetrating and the correlation [5] is similar to that in He~ and H3• However, there are offsetting H.F. and correlation errors that are s trongly de pe ndent on the geometry. The de pth of the H.F. attractive well should approximate the correlated one.…”