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