Interpolate the geometry of the stationary point (minimum or saddle point) by a quartic polynomial between the current and the previous geometry. If iflag=0 or no iflag is set, the next optimization step will be taken from the interpolated geometry using the interpolated energy and gradient. If iflag=1 the energy and gradient will be recalculated at the interpolated geometry before taking the new optimization step. Note though, that the additional effort of recalculating the energy and gradient is usually not met by the increase of the convergence rate of the optimization. thrlmin and thrlmax are min and max thresholds for the recalculation of the energy and the gradient in case iflag=1. I.e. the recalculation just takes place if the interpolated geometry isn't too close to the actual geometry thrlmin and isn't too remote from the actual geometry thrlmax. Default values are thrlmin=0.001 and thrlmax=0.05 in the scaled parameter space of the optimization.