Energy Jump

[cbeck]

Hello,

I have been running geometry optimizations on an orthorhombic ice Ih
unit cell with 8 water molecules (DZVP basis).

We are interested in obtaining the energy as a smooth function of the
two independent lattice parameters.  Using mgrid cutoff of 300 we find
a large (~.005 hartree) jump in the energy as a particular lattice
parameter value is crossed.

We suspect this has something to do with the set of density basis
functions selected by the mgrid cutoff changing at this point.  The
following two data sets demonstrate the energy jump.  Notice how one
of the PW_GRID total bounds changes by a large amount at the jump.

Lattice a (Angstrom)	  Lattice c (Angstrom)    Energy (Hartree)
Bounds (1,2,3)
SET 1
7.9			          7.5			            -137.58021900        {90., 48., 80}
7.96			          7.5			            -137.58014954        {90., 48., 80}
			        JUMP
8.02			          7.5			            -137.58580546	    {90., 54., 80}
8.08		                  7.5			            -137.58488193        {90.,
54., 80}

SET2
7.9			         7.5			           -137.58021990 	    {90., 48., 80}
7.9			         7.643			   -137.58019671         {90., 48., 80}
7.9			         7.786			   -137.58034585         {90., 48., 81}
			         JUMP
7.9			        7.929			   -137.58523364          {90., 48., 90}
7.9			        8.072			   -137.58366580	     {90., 48., 90}


Has anyone else run across this?  Is there a way to obtain smooth
output?  (like constraining the density basis if the hypothesis is
correct)

Corey