R12 methods: *R12

The calculation of MP2-R12 energy corrections is requested. Note that an integral-direct calculation must be carried out and that the key .R12 must be specified in the **INTEGRALS section.

.NO 1

Results for Ansatz 1 of the MP2-R12 method are not computed.

.NO 2

Results for Ansatz 2 of the MP2-R12 method are not computed.

.NO A

Results for approximation A of the MP2-R12 method are not printed.

.NO A'

Results for approximation A$^\prime$ of the MP2-R12 method are not printed.

.NO B

Results for approximation B are not computed and not printed.

.NO RXR

Those extra terms are ignored, which occur in approximation B when an auxiliary basis set is invoked for the resolution-of-identity (RI) approximation. If they are included, the results are marked as B$^\prime$.

.NO HYB

The default MP2-R12 calculation implemented in the Dalton program avoids two-electron integrals that involve two or more basis functions of the auxiliary basis (of course, only if such a basis is employed). This approach is denoted as hybrid scheme between approximations A and B. To obtain the full MP2-R12 energy in approximation B, the keyword .NO HYB must be specified. Then, two-electron integrals with up to two auxiliary basis functions are calculated but the calculation becomes more time-consuming.

.R12DIA

The MP2-R12 equations are solved by diagonalizing the matrix representation of the Fock operator in the basis of R12 double replacements. If negative eigenvalues occur, a warning is issued. When this happens, the results should not be trusted, since the RI approximation appears to be insuffiently accurate. This diagonalizing is the default.

.R12SVD

The MP2-R12 equations are solved by single value decomposition (the use of this keyword is not recommended).

.R12XXL

All possible output from the MP2-R12 approach is generated (the use of this keyword is not recommended).

.SVDTHR

READ (LUCMD,*) SVDTHR
Threshold for singular value decomposition (default = $10^{-12}$).

.VCLTHR

READ (LUCMD,*) SVDTHR
Threshold for neglect of R12 terms (default = 0, neglecting nothing).