THE CONVERGENCE OF ENERGY EXPANSIONS FOR MOLECULES IN ELECTROSTATIC FIELDS - A LINEAR-RESPONSE COUPLED-CLUSTER STUDY

The radii of convergence of power series expansions describing energy of a molecule in external electrostatic field are investigated using D 'Alembert ratio test, standard and generalized Cauchy-Hadamard criteri a, and Pade approximants. The corresponding coefficients at various fi eld and field-gradient components, representing multipole moments and (hyper)polarizabilities and including terms of tenth or even twentieth order, are determined using an ab initio linear response coupled-clus ter theory. Most calculations are performed for the HF molecule descri bed by the basis set of double zeta quality, while the role of basis s et is discussed by comparing the results with estimates of the radii o f convergence obtained with the basis set of [5s3p2d/3s2p] quality. Em phasis is placed on the dependence of the interval of convergence of p ower series expansion describing energy of a molecule in applied elect rostatic field on the nuclear geometry. The results might have importa nt implications for various numerical methods used to calculate electr ostatic molecular properties as functions of the internuclear geometry , including the finite-field and fixed-point-charge approaches.