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.

The spin-component scaled (SCS) and scaled opposite-spin (SOS) approaches are at the moment only available for MP2 calculations.

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 are used. (S. Grimme, J. Chem. Phys. 118 (2003) 9095.)

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.)