$IRC group (relevant for RUNTYP=IRC) This group governs the location of the intrinsic reaction coordinate (also called the minimum energy path, MEP), a steepest descent path in mass weighted coordinates, that connects the saddle point to reactants and products. The IRC serves a proof of the mechanism for a reaction, and is a starting point for reaction path dynamics. The IRC may be found for systems with QM atoms, EFP particles, or the combinations of QM and EFP particles, or QM plus the optional SIMOMM plug-in MM atoms. Restart data for RUNTYP=IRC is written into the PUNCH file. Information summarizing the reaction path is written to the TRAJECT file, which should be saved, appending these as various restarts are done. The graphics program MacMolPlt can display a movie of the entire mechanism, if you join the entire forward and entire backwards trajectory files, while changing the path distance parameter in the reverse part to a negative value. ----- there are five integration methods chosen by PACE. PACE = GS2 selects the Gonzalez-Schlegel second order method. This is the default method. Related input is: GCUT cutoff for the norm of the mass-weighted gradient tangent (the default is chosen in the range from 0.00005 to 0.00020, depending on the value for STRIDE chosen below. RCUT cutoff for Cartesian RMS displacement vector. (the default is chosen in the range 0.0005 to 0.0020 Bohr, depending on the value for STRIDE) ACUT maximum angle from end points for linear interpolation (default=5 degrees) MXOPT maximum number of constrained optimization steps for each IRC point (default=20) IHUPD is the hessian update formula. 1 means Powell, 2 means BFGS (default=2) GA is a gradient from the previous IRC point, and is used when restarting. OPTTOL is a gradient cutoff used to determine if the IRC is approaching a minimum. It has the same meaning as the variable in $STATPT. (default=0.0001) PACE = LINEAR selects linear gradient following (Euler's method). Related input is: STABLZ switches on Ishida/Morokuma/Komornicki reaction path stabilization. The default is .TRUE. DELTA initial step size along the unit bisector, if STABLZ is on. Default=0.025 Bohr. ELBOW is the collinearity threshold above which the stabilization is skipped. If the mass weighted gradients at QB and QC are almost collinear, the reaction path is deemed to be curving very little, and stabilization isn't needed. The default is 175.0 degrees. To always perform stabilization, input 180.0. READQB,EB,GBNORM,GB are energy and gradient data already known at the current IRC point. If it happens that a run with STABLZ on decides to skip stabilization because of ELBOW, this data will be punched to speed the restart. PACE = QUAD selects quadratic gradient following. Related input is: SAB distance to previous point on the IRC. GA gradient vector at that historical point. PACE = AMPC4 selects the fourth order Adams-Moulton variable step predictor-corrector. Related input is: GA0,GA1,GA2 which are gradients at previous points. PACE = RK4 selects the 4th order Runge-Kutta variable step method. There is no related input. ----- The next two are used by all PACE choices ----- STRIDE = Determines how far apart points on the reaction path will be. STRIDE is used to calculate the step taken, according to the PACE you choose. The default is good for the GS2 method, which is very robust. Other methods should request much smaller step sizes, such as 0.10 or even 0.05. (default = 0.30 sqrt(amu)-Bohr) NPOINT = The number of IRC points to be located in this run. The default is to find only the next point. (default = 1) ----- constraint ----- Of course, applying a constraint to the saddle point search and the reaction path means that you are not locating the true saddle, nor following the true reaction path. IFREEZ = array of Cartesian coordinates to freeze. The IRC stepper works in mass-weighted Cartesian space, making it impossible to freeze internal coordinates. An input of IFREEZ(1)=4,8 means to freeze the x coordinate of the 2nd atom and the y coordinate of the 3rd atom, that is, we count coordinates x1,y1,z1,x2,y2,z2,x3,y3,z3,... ----- The next two let you choose your output volume ----- Let F mean the first IRC point found in this run, and L mean the final IRC point of this run. Let INTR mean the internuclear distance matrix. NPRT = 1 Print INTR at all, orbitals at all IRC points 0 Print INTR at all, orbitals at F+L (default) -1 Print INTR at all, orbitals never -2 Print INTR at F+L, orbitals never NPUN = 1 Punch all orbitals at all IRC points 0 Punch all orbitals at F+L, only occupied orbitals at IRC points between (default) -1 Punch all orbitals at F+L only -2 Never punch orbitals ----- The next two tally the reaction path results. The defaults are appropriate for starting from a saddle point, restart values are automatically punched out. NEXTPT = The number of the next point to be computed. STOTAL = Total distance along the reaction path to next IRC point, in mass weighted Cartesian space. ----- The following controls jumping off the saddle point. If you give $HESS input, FREQ and CMODE will be generated automatically. SADDLE = A logical variable telling if the coordinates given in the $DATA deck are at a saddle point (.TRUE.) or some other point lying on the IRC (.FALSE.). If SADDLE is true, either a $HESS group or else FREQ and CMODE must be given. (default = .FALSE.) Related input is: TSENGY = A logical variable controlling whether the energy and wavefunction are evaluated at the transition state coordinates given in $DATA. Since you already know the energy from the transition state search and force field runs, the default is .F. FORWRD = A logical variable controlling the direction to proceed away from a saddle point. The forward direction is defined as the direction in which the largest magnitude component of the imaginary normal mode is positive. (default =.TRUE.) EVIB = Desired decrease in energy when following the imaginary normal mode away from a saddle point. (default=0.0005 Hartree) FREQ = The magnitude of the imaginary frequency, given in cm**-1. CMODE = An array of the components of the normal mode whose frequency is imaginary, in Cartesian coordinates. Be careful with the signs! You must give FREQ and CMODE if you don't give a $HESS group, when SADDLE=.TRUE. The option of giving these two variables instead of a $HESS does not apply to the GS2 method, which must have a hessian input, even for restarts. Note also that EVIB is ignored by GS2 runs. * * * * * * * * * * * * * * * * * * For hints about IRC tracking, see the 'further information' section. * * * * * * * * * * * * * * * * * * ========================================================== ==========================================================

generated on 7/7/2017