MOLPRO

46 BASIS SET EXTRAPOLATION 343
46 BASIS SET EXTRAPOLATION
Basis set extrapolation can be carried out for correlation consistent basis sets using
EXTRAPOLATE,BASIS=basislist,options
where basislist is a list of at least two basis sets separated by colons, e.g. AVTZ:AVQZ:AV5Z.
Some extrapolation types need three or more basis sets, others only two. The default is to use
n −3 extrapolation of the correlation energies, and in this case two subsequent basis sets and the
corresponding energies are needed. The default is not to extrapolate the reference (HF) energies;
the value obtained with the largest basis set is taken as reference energy for the CBS estimate.
However, extrapolation of the reference is also possible by specifying the METHOD R option.
The simplest way to perform extraplations for standard methods like MP2 or CCSD(T) is to use,
e.g.
***,H2O
memory,32,m
gthresh,energy=1.d-8
r = 0.9572 ang, theta = 104.52
geometry={O;
H1,O,r;
H2,O,r,H1,theta}
basis=avtz
hf
ccsd(t)
extrapolate,basis=avqz:av5z
table,basissets,energr,energy-energr,energy
head,basis,ehf,ecorr,etot
This will perform the first calculation with AVTZ basis, and then compute the estimated basis set
limit using the AVQZ and AV5Z basis sets. The correlation energy obtained in the calculation
that is performed immediately before the extrapolate command will be extrapolated (in this case
the CCSD(T) energy), and the necessary sequence of calculations [here HF;CCSD(T)] will be
automatically carried out.
The resulting energies are returned in variables ENERGR (reference energies), ENERGY (total
energies), and ENERGD (Davidson corrected energy if available); the corresponding basis sets
are returned in variable BASISSETS. The results can be printed, e.g., in a table as shown above,
or used otherwise. The above input produces the table
BASIS EHF ECORR ETOT
AVQZ -76.06600082 -0.29758099 -76.36358181
AV5Z -76.06732050 -0.30297495 -76.37029545
CBS -76.06732050 -0.30863418 -76.37595468
The extrapolated total energy is also returned in variable ECBS (ECBSD for Davidson corrected
energy if available).
In order to extrapolate the HF energy as well (using exponential extrapolation), three energies
are needed. One can modify the input as follows: