want to use 6-311++G(3d,3p)

[Hrishit Mitra]

I want to use 6-311++G(3d,3p) basis set for a optimization and frequency calculation using B3LYP method. What I have to mention in 'BASIS' section of input.

[Edoardo Aprà]

Both the NWChem internal library and https://www.basissetexchange.org/ don't have the 6-311++G(3d,3p) basis, but 6-311++G(3df,3pd), instead.

You should be able to obtain the 6-311++G(3d,3p)  basis from 6-311++G(3df,3pd) by removing the d polarization function on hydrogen and the f polarization function on the other atoms (see "Adding Polarization and Diffuse Functions" paragraph on https://gaussian.com/basissets/)