36.2.14 Numerical gradients (NUMERICAL)

NUMERICAL,active$_1$=step$_1$, active$_2$=step$_2$ ...;

With this option the gradients are computed by finite differences. step$_i$ is the increment for the active geometry parameter $active_i$. For active parameters which are not specified, the default values are used. By default, the increment is 0.01 bohr for bond distances and 0.5 or 1 degree for angles less than or greater than 90 degrees, respectively. These defaults can be modified by specifying RSTEP=value, ASTEP=value on the numerical card. Note that step sizes which are too large may lead to optimization failure. For each active variable, two additional energy calculations are necessary for each geometry optimization step - so this may be expensive! Symmetrical displacement coordinates are normally used to minimize the number of energy calculations. (see COORD keyword section 32.2.1).

For optimization of special energies see VARIABLE section 32.2.18.