In many cases the one-electron basis set can be specified in the simple form

`basis=`*name*

where *name* refers to the basis set name in the library. You can find all
possible basis sets on the MOLPRO Web page. Popular and often used for
high-level calculations are the correlation-consistent polarized basis sets
of Dunning and coworkers, denoted
`cc-pVDZ` (double zeta), `cc-pVTZ` (triple zeta), `cc-pVQZ` (quadruple
zeta), `cc-pV5Z` (quintuple zeta). These names can be used directly or
abbreviated by `VDZ`, `VTZ`, `VQZ` and so on. For computing certain
properties like electron affinities, polarizabilities, or intermolecular potentials
additional diffuse basis functions are needed, and such functions are included in the
augmented correlation-consistent basis sets, denoted
`aug-cc-pVDZ`, `aug-cc-pVTZ` etc. These names can be abbreviated by
`AVDZ`, `AVTZ`, `AVQZ` etc.

Another popular choice are the Gaussian basis sets of Pople and coworkers,
e.g. `6-31G**` (double zeta), `6-311G(2DF,2PD)` (triple zeta),
`6-311G++(2DF,2PD)` (augmented triple zeta) etc. It should be noted here
that by default MOLPRO uses spherical harmonics (, etc), while
`Gaussian` uses cartesian functions (, etc). This yields
slightly different energies. Cartesian functions
can also be used in MOLPRO, but then the keyword

`cartesian`

has to be given before the basis set specification.

It also possible to use different basis sets for different atoms. In this case the simplest input reads for example

`basis,c=vtz,o=avtz,h=vdz`

Basis sets can also be specified manually (exponents and contraction coefficients). Refer to the MOLPRO reference manual for details.

If no basis set is specified at all, MOLPRO uses `cc-pVDZ`, but this does not
mean that this is a sensible choice!

molpro@molpro.net 2019-03-21