CI wavefunction norm in Gaussian

    Dear fellow netters,
          I  am running CI(SD) calculations using G90, and  I
     am   trying  to  extract  the  coefficient  of  the   HF
     configuration. The manual says (page 165) that "Norm(A1)
     - 1 gives a measure of the correlation correction to the
     wavefunction." But what exactly is the  coefficient  for
     the  HF configuration? My guess is that it is  given  by
     1/Norm(A1), but I am not sure. Any comments?
          Thank you,
                    Gernot Frenking
                    frenking { *at * } dmrhrz11.bitnet
 Your guess is correct.  This is one of the most commonly asked questions,
 and the G92 manual has the equation for Norm(A) in an attempt to make the
 relationship clearer.  The wavefunction is stored in intermediate
 normalization.  That is:
 Psi(CISD) = Psi(0) + Sum(ia) Tia Psi(i-->a) + Sum (ijab) Tijab
 Psi(ij-->ab)
 Where Psi(0) is the Hartree-Fock determinant and has a coefficient of 1
 (that's what intermediate normalization means).  Norm(A) is the factor
 by which to divide the wavefunction as given above to fully normalize it.
 Thus
 Norm(A) = Sqrt (1 + Sum(ia) Tia*Tia + Sum(ijab) Tijab*Tijab)
 The coefficient of the Hartree-Fock determinant in the fully normalized
 wavefunction is then 1/Norm(A), the coefficient of singly excited
 determinant Psi(i-->a) is Tia/Norm(A), etc.
 Another pitfall to be careful of is that the sum over the doubly-excited
 determinants is really restricted to the unique determinants.  It is
 really
 Sum(i<j,a<b) Psi(ij-->ab ijab all alpha spin) +
 Sum(all ijab) Psi(ij-->ab i,a alpha j,b beta) +
 Sum(i<j,a<b) Psi(ij-->ab ijab all beta spin)
 For closed shell, the all beta coefficients are equal to the all alpha
 coefficients and if you request that the coefficients be printed (using
 IOP(9/28=N) to get coefficients larger than 10^-N or IOP(9/28=-1) to get all
 coefficients) only the alpha-alpha and alpha-beta coefficients will be
 printed.
 Mike Frisch
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