A new approach for developing of basis sets to be used along with effective core potential is systematically studied. The behavior of the LCAO coefficients versus the ln(alpha) of the respective primitives can provide simple guidelines to establish the range over which the basis set should be developed or modified, especially when using effective core potential. Double-zeta basis sets were modeled for SBK pseudopotential from all-electron basis sets for a series of compounds containing elements of the second period of the periodic table. Application of the modeled basis sets at the Hartree-Fock and MP2 levels of theory shows that the new method provides molecular properties as accurate as those calculated by all-electron calculations. (C) 1997 John Wiley & Sons,Inc.181519181929