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Spin-component scaling approaches (SCS/SOS)

By introducing individually scaling factors to the same-spin and opposite-spin contributions of the correlation energy second-order methods can be modified for a (hopefully) better performance. SCS-MP2 has first been proposed by S. Grimme and SOS-MP2 by Y. Jung et al. (see below). The generalization of SCS and SOS to CC2 for ground and excited states is described in [16]. It uses the same scaling factors as proposed for the original SCS- and SOS-MP2 approaches (see below). In the ricc2 program we have also implemented SCS and SOS variants of CIS(D) for excitation energies and of CIS(D) and ADC(2) for excitation energies and gradients, which are derived from SCS-CC2 and SOS-CC2 in exactly the same manner as the unmodified methods can be derived as approximations to CC2 (see Sec. 9.2 and ref. [116]). Please note, that the SCS-CIS(D) and SOS-CIS(D) approximations obtained in this way and implemented in ricc2 differ from the spin-component scaled SCS- and SOS-CIS(D) methods proposed by, respectively, S. Grimme and E. I. Ugorodina in [117] and Y. M. Rhee and M. Head-Gordon in [118].

A line with scaling factors has to be added in the $ricc2 data group:

$ricc2
scs  cos=1.2d0  css=0.3333d0
cos denotes the scaling factor for the opposite-spin component, css the same-spin component.

As an abbreviation
scs
can be inserted in $ricc2. In this case, the SCS parameters cos=6/5 and css=1/3 proposed S. Grimme (S. Grimme, J. Chem. Phys. 118 (2003) 9095.)are used. These parameters are also recommended in [16] for the SCS variants of CC2, CIS(D), CIS(D), and ADC(2) for ground and excited states.

Also, just
sos
can be used as a keyword, to switch to the SOS-MP2 approach proposed by the Head-Gordon group with scaling factors of cos=1.3 and css=0.0 (Y., Jung, R.C. Lochan, A.D. Dutoi, and M. Head-Gordon, J. Chem. Phys. 121 (2004) 9793.), which are also recommended for SOS variants of CC2, CIS(D), CIS(D), and ADC(2). The Laplace-transformed algorithm for the SOS variants are activated by the additional data group $laplace:

$laplace
   conv=4
For further details on the Laplace-transformed implementation see Sec. 8.6.



Subsections
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Next: Restrictions: Up: Second-Order Approximate Coupled-Cluster (CC2) Previous: Parallel RI-MP2 and RI-CC2   Contents   Index
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