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