Size of the CI Matrix

What is a typical size of the CI matrix Consider a small system, H2O with a 6-31G(d) basis. For the purpose of illustration, let us for a moment return to the spinorbital description. There are 10 electrons and 38 spin-MOs, of which 10 are occupied and 28 are empty. There are possible ways of selecting n electrons out of the 10 occupied orbitals, and K2s,n ways of distributing them in the 28 empty orbitals. The number of excited states for a given excitation level is thus Kiq. K2s,n, and the total number of... 

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