1% 2% $Id$ 3% 4\label{sec:mepgs} 5 6 7The MEPGS module performs a search for the two critical points on the 8potential energy surface connected to a saddle point of the molecule 9defined by input using the \verb+GEOMETRY+ directive (see Section \ref{sec:geom}). 10The algorithm programmed in MEPGS is a {\bf constrained} trust region quasi-newton optimization and approximate energy Hessian updates. 11 12 13Optional input for this module is specified within the compound 14directive, 15\begin{verbatim} 16 MEPGS 17 18 OPTTOL <real opttol default 3e-4> 19 20 EPREC <real eprec default 1e-7> 21 22 STRIDE <real stride default 0.1> 23 24 EVIB <real evib default 1e-4> 25 26 MAXMEP <integer maxiter default 250> 27 28 MAXITER <integer maxiter default 20> 29 30 INHESS <integer inhess default 2> 31 32 (BACKWARD || FORWARD) <string default both> 33 34 (MSWG || NOMSWG) <string default NOMSWG> 35 36 (XYZ || NOXYZ) 37 38 END 39\end{verbatim} 40 41\sloppy 42 43\section{Convergence criteria} 44 45The user may request a specific value for the tolerance with the keyword \verb+OPTTOL+ which will couple all the convergence criteria in the following way: 46 47\begin{verbatim} 48 GRMS 1.0*OPTTOL 49 GMAX 1.5*OPTTOL 50 XRMS 4.0*OPTTOL 51 XMAX 6.0*OPTTOL 52\end{verbatim} 53 54\section{Available precision} 55 56\begin{verbatim} 57 EPREC <real eprec default 1e-7> 58\end{verbatim} 59 60In performing a constrained trust region optimization the precision of the energy is coupled to the convergence criteria (see Section \ref{sec:tropt:eprec}). 61Note that the default EPREC for DFT calculations is 5e-6 instead of 1e-7. 62 63\section{Controlling the step length} 64 65\begin{verbatim} 66 STRIDE <real stride default 0.1> 67\end{verbatim} 68 69A dynamic stride (\verb+stride+) is used to control the step length during 70the minimum energy path walking when taking the Euler step as starting point. 71 72\section{Moving away from the saddle point} 73 74\begin{verbatim} 75 EVIB <real evib default 1e-4> 76\end{verbatim} 77 78The expected decrease in energy (\verb+evib+) assuming a quadratic approximation around the saddle structure to be obtained. 79 80\section{Maximum number of MEPGS steps} 81 82\begin{verbatim} 83 MAXMEP <integer maxmep default 250> 84\end{verbatim} 85 86By default at most 250 minimum energy path steps will be taken, 87but this may be modified with this directive. 88 89\section{Maximum number of steps} 90 91\begin{verbatim} 92 MAXITER <integer maxiter default 20> 93\end{verbatim} 94 95By default at most 20 {\bf constrained} geometry optimization steps will be taken, 96but this may be modified with this directive. 97 98\section{Initial Hessian} 99\begin{verbatim} 100 INHESS <integer inhess default 2> 101\end{verbatim} 102 103With this option the MEPGS module will be able to transform 104Cartesian Hessian from previous frequency calculation. 105 106\section{Selecting the side to traverse} 107\begin{verbatim} 108 (BACKWARD || FORWARD) <string default both> 109\end{verbatim} 110 111With this option the MEPGS module will select which side of the 112minimum energy path to explore. By default both sides are explored 113for a MEPGS run. 114 115\section{Using mass} 116\begin{verbatim} 117 (MSWG || NOMSWG) <string default NOMSWG> 118\end{verbatim} 119 120With this option the MEPGS will trigger the use of mass when following the 121minimum energy path. Mass is not used as default, if mass is used then 122this formally becomes an intrinsic reaction coordinate. 123 124\section{Minimum energy path saved XYZ file} 125 126\begin{verbatim} 127 XYZ [<string xyz default $fileprefix>] 128 NOXYZ 129\end{verbatim} 130 131The \verb+XYZ+ directive causes the geometry at each calculated 132structure on the minimum energy path to be output into file in the 133permanent directory in XYZ format. 134The optional string will 135prefix the filename. The \verb+NOXYZ+ directive turns this off. 136 137For example, the input 138\begin{verbatim} 139 mepgs; xyz ; end 140\end{verbatim} 141will cause a trajectory file filename.xyz to be created 142in the permanent directory. 143 144\section{MEPGS usage} 145 146\begin{verbatim} 147 start somename 148 geometry; <saddle point body > ; end 149 task theory freq 150 freq; reuse somename.hess ; end 151 mepgs; <mepgs options> ; end 152 task theory mepgs 153\end{verbatim} 154 155In the above example, after performing a frequency analysis for the saddle 156point, the information of the force constant matrix is reused (freq directive) 157in order to be able to follow the transition state mode. 158 159\fussy 160 161 162