12 Integral-direct calculations

With the exception of perturbative triples corrections, all methods implemented in MOLPRO can be performed in integral-direct mode, i.e., without storage of the two-electron integrals on disk. Naturally, this takes longer than conventional calculations, since the integrals are recomputed whenever needed. The integral-direct mode is activated by giving the gdirect directive before the first energy calculation, e.g.,

***,formaldehyde
print,basis,orbitals   !this is optional: print the basis set and the
                       !occupied orbitals
geometry               !define the nuclear coordinates
ang
C
O  , C , rco
H1 , C , rch , O , hco
H2 , C , rch , O , hco , H1 , 180
end

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

gdirect                !integral-direct calculation
basis=vdz              !Select basis set
hf                     !Perform HF calculation
mp2                    !Perform MP2 calculation