Quasiparticle, virtual orbital space

Electron density difference matrices that correspond to the transition energies in the EP2 approximation may be used to obtain a virtual orbital space of reduced rank [27] that introduces only minor deviations with respect to results produced with the full, original set of virtual orbitals. This quasiparticle virtual orbital selection (QVOS) process provides an improved choice of a reduced virtual space for a given EADE and can be used to speed up computations with higher order approximations, such as P3 or OVGF. Numerical tests show the superior accuracy and efficiency of this approach compared to the usual practice of omission of virtual orbitals with the highest energies [27],... 

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