The energies and lifetimes (i.e., inverse decay rates) of resonance (quasibound) states in chemical reactions are associated with the complex eigenvalues of the complex scaled Hamiltonian. The corresponding eigenfunctions are square integrable and are compact, localized functions in the coordinate space. Complex scaling is applicable when the potential, V(x), is dilation analytic. Ab initio potentials, however, are given on a grid, V-n= V(x(n)). Starting from the theoretical work of Moiseyev and Hirschfelder [J. Chem. Phys. 88, 1063 (1988)], we propose an efficient numerical method of calculating V(x(n) exp(i theta)) by acting on the unsealed potential with a grid represented scaling operator (S) over cap.