In Geometry Optimizations and Transition State searches, the gradients at the initial geometry may be quite large and in such case there is no need to apply the same high integration precision as may be required close to convergence when the gradients become very small. Therefore, one may set parameters to reduce the numerical integration accuracy during optimization to save time, and use the high precision only in the final stage of the optimization.
The program starts with an initial value accfirst, to get an assessment of the (local) gradients. Subsequently it adjusts it according to the progress towards convergence. All values are kept between a lower and an upper bound: accmin and accmax respectively.
All three parameters, accfirst, accmin, accmax, are controlled by the key INTEGRATION.
The simplest application, discussed above, specifies one value (acc1). This defines all three parameters the same value: the upper bound (accmax), the first value (accfirst), and the lower bound (accmin). Note that this default behavior in case 1 value is supplied is different than it was in ADF2010.
If two values are supplied, the smallest is taken for the lower bound, the larger for the upper bound. The value for the first cycle equals the upper bound.
If three values are supplied, the smallest is the lower bound, the largest the upper bound, the remaining value is used to start with.