Subject: Question about density matrices in g98

From: Wong Lai Ho <alarry-.at.-yangtze.hku.hk>
Date: Thu, 2 Oct 2003 12:25:26 +0800 (HKT)

Dear cclers,
Do anyone know why some off-diagonal elements of some density matrices 
trucated to zero during SCF cycle in gaussian calculations? Here is an 
example of output file.

Cm= 
 Alpha MO coefficients at cycle   1.
            1            2            3            4
   1 -0.225856D+00 0.225856D+00  0.911792D+00  0.911792D+00
   2 -0.537684D+00 0.537684D+00 -0.770242D+00 -0.770242D+00
   3  0.225856D+00 0.225856D+00  0.911792D+00 -0.911792D+00
   4  0.537684D+00 0.537684D+00 -0.770242D+00  0.770242D+00
P=
 Alpha density matrix  at cycle   1.
             1             2             3             4
   1  0.510110D-01
   2  0.121439D+00  0.289104D+00
   3  0.000000D+00  0.000000D+00  0.510110D-01
   4  0.000000D+00  0.000000D+00  0.121439D+00  0.289104D+00

According to 
P(i,j)=Sum[Cm(i,k)*Cm'(j,k)]  k=1,...,No.of electrons/2

The (3,1) elments in density matrix P should be
P(3,1)= Cm(3,1)*Cm'(1,1) k=1 (two electrons/2)
      =0.225856D+00*-0.225856D+00
      =-0.510110D-01 
Do I make something wrong above? 
Any comment will be appreciated.

Larry