A theoretical study of the radiative lifetime of the CH A 2Δ state

Abstract

The complete active space (CASSCF) method has been used to generate wave functions for calculations of the A ²Δ–X ²Π transition moment. This transition provides an interesting test case for different ab initio methods since the experimental data for the A state radiative lifetime should be very accurate. The two most accurate recent experiments agree to within 1% of each other, with a value of 536 ns. The best earlier theoretical value is 397 ns. Our calculations show an extremely slow convergence with basis sets and active spaces. A calculation with a basis set of more than quadruple zeta quality with two d and one f function on carbon and three p and one d function on hydrogen gave an error of 20% (446 ns compared to 536 ns) for the lifetime, even when a large active space of 5σ,3π, and 1δ orbital was used. Our best calculated value 525 ns finally agreed very well with the experimental value, and is consequently larger compared to the earlier theoretical value by more than 30%.