Error in MM+ Calculations

An obscure bug has been found in HyperChem. The effect is that under certain circumstances, HyperChem reads a set of parameters for one torsional angle calculation, and uses the same parameters for another torsional angle as well, instead of getting new parameters. This happens only when:

* The MM+ force field is used
* The 'default' parameters are not used -- i.e., all atoms in a dihedral angle have types

* The workspace contains dihedral angles with the same atom types arranged in two different ways: A-B-C-D and A-C-B-D.

This is, of course, a problem only if the parameters for the two different angles should be different. This is the case for 80 pairs of torsional parameters out of 1498 sets of parameters. In most cases, even if the error occurs it does not lead to much change in the parameters; but in the case of cyclohexa-1,4-diene, there is a significant change in the parameter values (including a sign change), and there are many incorrect parameters which overwhelm the effects of the correct ones.

A list of the torsional parameters for which this occurs is available here. We would like to thank Alexandre Hocquet for bringing the problem to our attention.

To download a corrected version of the MM+ backend (137 kB):

For HyperChem 4 for Windows, click here.

For HyperChem 4.5 for Windows, click here.

The above are self-extracting compressed files; just copy the appropriate one to your HyperChem directory and then run it (for example, by double-clicking on the file's icon in the Windows File Manager). The updated MMPLUS.EXE should over-write the old one. The file for HyperChem 4 will also work for HyperChem 3, though the software will complain about the version mismatch.

For HyperChem 4.5 for SGI, click here.

After downloading the SGI version of mmplus, de-compress it with 'uncompress', and place it in the directory /usr/sbin (replacing the original file). The file's permissions should allow execution; you may need to correct them manually with "chmod a+x hc45_mmplus".

For further information send email to support@hyper.com

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