18 EFFECTIVE CORE POTENTIALS

Pseudopotentials (effective core potentials, ECPs) may be defined at the beginning of BASIS blocks.

The general form of the input cards is

ECP,atom,[ECP specification]

which defines a pseudopotential for an atom specified either by a chemical symbol or a group number. The ECP specification may consist either of a single keyword, which references a pseudopotential stored in the library, or else of an explicit definition (extending over several input cards), cf. below.