gaussian基本概念和用法(17)

The grid may too coarse; the default grid is 'fine', to specify a finer grid use the keyword 'Integral(grid=ultrafine)' You may need to use a different algorithm e.g. GDIIS instead of RFO.

Inexpensive Integrals

The message/warning from Gaussian reads \repeating last cycle.\this message only indicates that the program is switching to a more accurate integral evaluation. During the initial stages of direct scf, integrals are calculated with pretty loose tolerances and cut-offs to speed things up. However, this can lead to problems sometimes. Gaussian detects this and tightens up the integral evaluation if necessary. So the program just tells you that the cheap way of doing it didn't work and that it is therefore switching to the more accurate (but more time-consuming) method. This happens often. It is possible that the geometry optimization took a bad step, or maybe it is the first point of the optimization. The problem is probably related to the geometry, if the geometry looks reasonable you could try using \which uses the full accuracy integrals from the beginning or use \method.

Geometry Optimizations

It is often assumed that, given the same starting geometry, then any program that has a choice as to how the geometry is entered should (in the end) produce the same optimized final geometry. If all calculations were done to infinite precision and all geometry optimizations were continued until the forces were exactly zero, and all optimizations used analytic first AND SECOND derivatives then optimizations starting from the same structure but using different coordinate systems would go to exactly the same place. In fact, because of differences in the Hessians, two optimizations with different coordinates started at the same point far from any minimum might fall down into different wells and to different minima. (This will not be the case if the two optimizations use analytic rather than approximated second derivatives at every point -- then every step should be the same regardless of coordinate system.) All of these considerations apply to comparing Cartesian with Internal coordinates and also to comparing two different sets of Internal coordinates.

How To Produce A Potential Energy Surface

Start with the .log file for the optimized molecule

1.Go to 'view', 'show labels' and then 'view', 'z-matrix editor'. 2.Select 'opt none' and all numbers in Opt columns will become 0

3.Find the dihedrals that you want to vary and type 1 in opt3 column. Make sure that the carbons and hydrogens etc.

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are connected in the correct way, see diagram below;

Original z-matrix.

New z-matrix.

4.In the main window select 'calculate', 'gaussian' and then 'scan'. Select model chemistry and basis set as normal, on warning select OK. If symmetry will break during scan process then check the 'ignore symm' box.

5.Edit - then check that all variables are frozen except for the ones that you want to vary, i.e. D1, D2. Find these at the bottom of the molecular specification, then; D1 180. 18 -10. 18 = how many steps, -10. = size of each step and which direction. Do the same for D2. Don't forget the 'full stops'. Save, close, submit.

How To Get A Good Guess For FC

Set up molecule and do a freq=noraman job at a low level of theory, hf/3-21g is okay. These frequencies are meaningless but the second derivatives provide a good start for the Hessian. You can arrange for this to be performed automatically by firstly setting up the calculation as normal then; 1.Go to 'edit' and bottom of input 2.Leave a blank line then; o--link1--

o%chk='same filename as given for freq job' o%mem='same mem as above'

o#p opt=rcfc geom=allcheck guess=read b3lpy/6-31g* 3.blank line 4.Save, submit.

To Add Extra Basis Functions To Only Certain Specified Atoms

1.View labels on molecule and make a note of those that you want to add extra basis functions to.

2.Go to 'calculate', 'gaussian' and set up as usual but add keyword 'extrabasis', it's not necessary to select 'write connectivity'.

3.Edit - go to end of molecular specification and type

6 9 31 32 etc (the labels of the molecules). Make sure to end line with 0 d 1 1.0 0.8 1.0 **** blank line

This is all to add 'd' functions onto the specified atoms. Save, submit.

You can also use opt=readfc to get the fc from the .chk file.

Using freqchk

Use freqchk to do isotopic substitutions, change pressure, temp etc.

The first thing to do is to move a copy of the .chk file into the G98W directory. Then open 'notepad' and write a script of the type shown below;

For something like methyl acetate you might have a file named meth.dat containing the following lines; meth.chk

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n 298.15 1 0 n 12 16 16 12 2 2 2 12 2 2 2

This script will deuterate all the hydrogens of the methyl groups. The first \Then give the temperature, pressure and 0 takes the default scaling. The second \common isotopes and then you provide one line per atom with the mass numbers. The program will look up the exact mass and use that in the calculation.

Save this file in the G98W directory also.

Open a 'command prompt' window and change to the G98W directory Type 'freqchk meth.frq' A new file will be created with the new data in it.

Visualizing Molecular Orbitals and Potential Energy Surfaces. These operations can be done in a variety of different ways.

You can view them both in GaussView using the .chk file and the 'surfaces' option in 'results'.

To view the potential energy surface, generate the ESP first, then the Total Density and then use the ESP and map on the Total Density.

The disadvantage of this is that it is quite slow and the file cannot be saved. You can print off a picture but once you close the file then you have …… 此处隐藏：7064字，全部文档内容请下载后查看。喜欢就下载吧 ……