# Grid Calculation

The GRID calculation is the two-dimensional analog of the PATH calculation. In a PATH calculation, one coordinate is flagged with a '-1'. In a GRID calculation, two coordinates are flagged by '-1's. An example of a GRID calculation is shown in the Figure. Note that the keywords STEP1=n.nn and STEP2=m.mm are essential.

 SYMMETRY STEP1=0.01 STEP2=1 Water, potential energy surface for H O 0.92 -1 0 0 0 0 1 H 0.92 0 104 -1 0 0 2 1 2 1 3

In this example, the potential energy surface for water is generated. For one axis of the 2-D plot, the O-H bond length is varied from 0.92Å to 1.02Å, in 11 steps of 0.01Å, and in the other axis, the H-O-H angle is varied from 104 to 114 in 11 steps of 1.0 degree. Because of the use of symmetry, there are no variables to be optimized. If symmetry were not used, then the second O-H bond length could either be optimized, by setting its flag to 1, or held constant, by setting its flag to 0.

Keywords used with the GRID option are POINT1=n and POINT2=m: to use n and m points in directions 1 and 2; and keywords to specify how the geometry should be optimized. n x m should be less than 400000