The theoretical investigations of third-harmonic susceptibilities of lithium formate monohydrate crystals with chemical formula of HCOOLi o H_2O have been performed for the first time, by using the time-dependent coupled-perturbed Hartree-Fock (CPHF) method, the finite-field MP2 approach and the oriented-gas approximation at both molecular and crystalline levels. It has been found that the elements ofγ(3ω；ω,ω,ω) tensors of supermolecules depend linearly on chain lengths. The calculated magnitudes of X(-3ω；ω,ω,ω) tensor components of lithium formate monohydrate crystals are influenced by electron correlation, frequency, basis set and the local field factor, of which electron correlation is the most important and frequency dispersion is negligible at the off-resonant frequencies. Finally, the third-harmonic coefficients have been estimated.

Key words: NON LINEAR OPTICS, lithium formate, POLARIZABILITIES, HARMONIC ANALYSIS