30.12 DF-LMP2-F12 calculations with local approximations

Local variants of the DF-MP2-F12 methods are invoked by the commands DF-LMP2-F12 with ansatz 3*A(loc). These calculations are performed with a different program than non-local calculations, and currently do not include the perturbative CABS singles correction (since CABS is not used in this program). The (loc) option implies that the LMP2 calculation with domain approximations is performed, and by default a local projector as described in H.-J. Werner, J. Chem. Phys. (submitted, 2008) is used. This yields very similar energies as the DF-LMP2-F12/3*A(D,NOX) method with full domains at lower cost, and the method can be applied to quite large molecules.

The local projector can also be used with other ansätze, by specifying, e.g. ANSATZ=3C(loc), but in this case the local projection is only simulated and does not save any time.

Special options for these local variants are (ansatz 3*A only):

PAIRS

Specifies which pairs to be treated by R12 or F12
(STRONG|CLOSE|WEAK|ALL; pairs up to the given level are included). The default is ALL. Note that even with ALL very distant pairs are neglected if these are neglected in the LMP2 as well.

USEVRT

If zero, the $1-pp+po+op-p'o-op'$ form of the projector is used, and local RI approximations apply to $p$ and $p'$. If set to 1, the $1+oo-vv-p'o-op'$ form of the projector is used; any local RI approximation then applies only to the RI contribution $p'$.

USEPAO

If USEPAO=1 use pair-specific local projectors instead of $vv$. This is appropriate and the default if domain approximations are made in the LMP2 (DOMSEL<1). USEPAO=1 automatically implies USEVRT=1, i.e. local RI approximations only affect the RI contributions $p'$.

FULLAO

if USEVRT=0 and FULLAO=1, local RI approximations only apply to the RI contributions $p'$. This should give the same results as USEVRT=1 (only applies if USEPAO=0).

DEBUG

Parameter for debug print

LOCFIT_F12

If set to one, use local fitting. Default is no local fitting (LOCFIT_F12=0)

LOCFIT_R12

Alias for LOCFIT_F12. Local fitting is not recommended in R12 calculations.

FITDOM

Determine how the base fitting domains are determined (only applies if LOCFIT_F12=1):
0: Fitdomains based on united operator domains;
1: Fitdomains based in orbital domains;
2: Fitdomains based on united pair domains using strong pairs;
3: Fitdomains based on united pair domains using strong, close and weak pairs (default);

RDOMAUX

Distance criterion for density fitting domain extensions in case of local fitting. The default depends on FITDOM.

IDOMAUX

Connectivity criterion for density fitting domain extensions in case of local fitting.

RAODOM

Distance criterion for RI domain extensions. Zero means full RI basis (default). If USEPAO=1 or USEVRT=1 or FULLAO=1 a value of 5 bohr is recommended. In other cases the local RI domains must be very large (RAODOM>12) and the use of local RI approximations is not recommended.

IAODOM

Connectivity criterion for RI domain extensions. Zero means full RI basis (default). Values greater or equal to 2 should lead to sufficiently accurate results, provided the local projector (USEPAO=1) is used.

THRAO

Screening threshold for coulomb integrals in the AO or RI basis.

THRF12

Screening threshold for F12 integrals.

THRMO

Screening threshold for half transformed integrals.

THRPROD

Product screening threshold in the first half transformation.

NOMP2

If set to 1, only the F12 calculation is performed, and the LMP2 is skipped. This is sometimes useful if full domains are used, since the iterative LMP2 then causes a big overhead and needs a lot of memory. It is then more efficient to do the a DF-MP2 calculation separately and compute the total energy as the sum of the DF-MP2 energy and the F12 energy

Further options for density fitting are described in section 11, and further options to choose the ansatz in section 30.9.

Typical inputs for calculations with local approximations are:

 
!parameters for local density fitting:
DFIT,LOCFIT_F12=1,FITDOM_MP2=1,IDOMAUX_MP2=3,DSCREEN=1     
!LMP2-F12(loc) with local RI:
{DF-LMP2-F12,ANSATZ=3*A(LOC),DOMSEL=0.985,RAODOM=5,PAIRS=WEAK}

This would perform a local MP2 with a Boughton-Pulay domain completeness criterion of 0.985. In the F12 part, distant pairs are not included (PAIRS=WEAK) and the local projector is used (USEPAO=1, default). Local density fitting and local RI approximations are used.

A corresponding non-local calculation (still using localized orbitals and the diagonal ansatz) would be

{DF-LMP2-F12,ANSATZ=3*A(LOC),DOMSEL=1.0,USEVRT=1,NOMP2=1}
ecorr_F12=ef12   

{DF-MP2}
ecorr_MP2=energy-energr             !mp2 correlation energy
ecorr_MP2_F12=ecorr_MP2+ecorr_F12   !total correlation energy

Note: The use of local DF and RI domains is still experimental and should be use with care!