Sanibel Symposium

Single reference coupled cluster methods are well established that it gives the most accurate results for most non-degenerate closed-shell ground states. However, it can be inadequate when the zeroth order wavefunction is dominated by more than one slater determinant. Here, we explore the possibility to extend SR-CC theory for excited states and open shell molecular systems by fixing the appropriate amplitudes to the values determined by spin or spatial symmetry. This scheme is expected to accelerate the process of approaching the exact solution which is generally achieved by including higher cluster operators. We report the preliminary results for systems having two active electrons. As an example we consider the excited states of ozone and lower lying states of the acetylene dication.