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18.5 Basis sets

In MOLPRO the F12 integrals can only be computed using density fitting (DF) approximations. The many electron integrals are approximated by resolutions of the identity (RI) expansions. Thus, F12 calculations require three different basis sets: the orbital (AO) basis, the DF basis, and the RI basis.

We recommand as AO basis sets the augmented correlation consistent basis sets (denoted AVnZ) or the specially optimized correlation consistent F12 basis sets (denoted VnZ-F12, cf. K.A. Peterson, T.B. Adler, and H.-J. Werner, J. Chem. Phys. 128, 084102 (2008)). Normally, triples zeta basis sets (AVTZ or VTZ-F12) yield excellent results that are close to the basis set limit. Diffuse basis functions are rather essential both for the HF and MP2-F12 energies, and therefore the standard VTZ sets are not recommended. If the AVnZ or VnZ-F12 orbital basis sets are used, suitable density fitting (DF) basis and resolution of the identity (RI) basis sets are automatically chosen. For the AV$nZ$ basis sets, the AVnZ/MP2FIT amd VnZ/JKFIT basis sets are used for the DF and RI, respectively. For the V$n$Z-F12 orbital basis sets, the correspondint OPTRI CABS basis sets are used by default. Other basis sets can be chosen using the DF_BASIS and RI_BASIS options (cf. section 18.1). See section 16 for more details about density fitting.

The following example shows a ccsd(t)-f12 calculation using the VTZ-F12 basis of Peterson et al., J. Chem. Phys. 128, 084102 (2008), along with the associated RI basis set of Yousaf and Peterson, J. Chem. Phys. 129, 18410 (2008):

***,formaldehyde
geometry={
C
O  , C , rco
H1 , C , rch , O , hco
H2 , C , rch , O , hco , H1 , 180
}

rco=1.182 Ang
rch=1.102 Ang
hco=122.1789 Degree

basis=vtz-f12
hf
ccsd(t)-f12,ri_basis=vtz-f12/optri,df_basis=avtz
e_F12a=energy(1)   !save F12a energy in variable e_F12a
e_F12b=energy(2)   !save F12b energy in variable e_F12b

The given RI and DF basis sets would also be used by default. With the AVTZ orbital basis the input would read

basis=avtz
hf
ccsd(t)-f12,ri_basis=avtz/optri,df_basis=avtz

In this case the default would be (for historical reasons and backward compatibility)

basis=avtz
hf
ccsd(t)-f12,ri_basis=vtz/jkfit,df_basis=avtz



Next: 18.6 Symmetry Up: 18 Explicitly correlated methods Previous: 18.4 The correlation factor   Contents   PDF

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