$GRADEX group (optional, for RUNTYP=GRADEXTR) This group controls the gradient extremal following algorithm. The GEs leave stationary points parallel to each of the normal modes of the hessian. Sometimes a GE leaving a minimum will find a transition state, and thus provides us with a way of finding that saddle point. GEs have many unusual mathematical properties, and you should be aware that they normally differ a great deal from IRCs. The search will always be performed in cartesian coordinates, but internal coordinates along the way may be printed by the usual specification of NZVAR and $ZMAT. METHOD = algorithm selection. SR A predictor-corrector method due to Sun and Ruedenberg (default). JJH A method due to Jorgensen, Jensen and Helgaker. NSTEP = maximum number of predictor steps to take. (default=50) DPRED = the stepsize for the predictor step. (default = 0.10) STPT = a flag to indicate whether the initial geometry is considered a stationary point. If .TRUE., the geometry will be perturbed by STSTEP along the IFOLOW normal mode. (default = .TRUE.) STSTEP = the stepsize for jumping away from a stationary point. (default = 0.01) IFOLOW = Mode selection option. (default is 1) If STPT=.TRUE., the initial geometry will be perturbed by STSTEP along the IFOLOW normal mode. Note that IFOLOW can be positive or negative, depending on the direction the normal mode should be followed in. The positive direction is defined as the one where the largest component of the Hessian eigenvector is positive. If STPT=.FALSE. the sign of IFOLOW determines which direction the GE is followed in. A positive value will follow the GE in the uphill direction. The value of IFOLOW should be set to the Hessian mode which is parallel to the gradient to avoid miscellaneous warning messages. GOFRST = a flag to indicate whether the algorithm should attempt to locate a stationary point. If .TRUE., a straight NR search is performed once the NR step length drops below SNRMAX. 10 NR step are othen allowed, a value which cannot be changed. (default = .TRUE.) SNRMAX = upper limit for switching to straight NR search for stationary point location. (default = 0.10 or DPRED, whichever is smallest) OPTTOL = gradient convergence tolerance, in Hartree/Bohr. Used for optimizing to a stationary point. Convergence of a geometry search requires the rms gradient to be less than OPTTOL. (default=0.0001) HESS = selection of the initial hessian matrix, when STPT=.TRUE. = READ causes the hessian to be read from a $HESS. = CALC causes the hessian to be computed. (default) ---- the next parameters apply only to METHOD=SR ---- DELCOR = the corrector step should be smaller than this value before the next predictor step is taken. (default = 0.001) MYSTEP = maximum number of micro iteration allowed to bring the corrector step length below DELCOR. (default=20) SNUMH = stepsize used in the numerical differentiation of the Hessian to produce third derivatives. (default = 0.0001) HSDFDB = flag to select determination of third derivatives. At the current geometry we need the gradient, the Hessian, and the partial third derivative matrix in the gradient direction. If .TRUE., the gradient is calculated at the current geometry, and two Hessians are calculated at SNUMH distance to each side in the gradient direction. The Hessian at the geometry is formed as the average of the two displaced Hessians. If .FALSE., both the gradient and Hessian are calculated at the current geometry, and one additional Hessian is calculated at SNUMH in the gradient direction. The default double-sided differentiation produces a more accurate third derivative matrix, at the cost of an additional wave function and gradient. (default = .TRUE.) ========================================================== * * * * * * * * * * * * * * * * * * * See the 'further information' section for some help with GRADEXTR runs. * * * * * * * * * * * * * * * * * * * ==========================================================

generated on 7/7/2017