The general expressions we recently derived for calculating internal conversion decay rate constants between two adiabatic and between two diabatic displaced-distorted-rotated harmonic potential energy surfaces (including all vibrational modes) are now applied to determine the decay rate constants of (1)B(2)u(pi pi*)-> B-1(3u)(n pi*), B-1(2u)(pi pi*)-> Ag-1, and B-1(3u)(n pi*)->(1)A(g) internal conversions in pyrazine molecule. The minimal models with respect to the number and the types of vibrational modes are investigated for these processes. An exact expression for the adiabatic vibrational frequencies of the coupling modes in terms of interstate coupling constants and the parameters determining the diabatic potential energy surfaces are also derived.