  We evaluated the individual atom contributions to the second harmonic
generation (SHG) coefficients of LiCs2PO4 (LCPO) by introducing the partial
response functionals on the basis of first principles calculations. The SHG
response of LCPO is dominated by the metal-cation-centered groups CsO6 and
LiO4, not by the nonmetal-cation-centered groups PO4 one expects from the
existing models and theories. The SHG coefficients of LCPO are determined
mainly by the occupied orbitals O-2p and Cs-5p as well as by the unoccupied
orbitals Cs-5d and Li-2p. For the SHG response of a material, the polarizable
atomic orbitals of the occupied and the unoccupied states are both important.
