The descriptor file contains a description of the force field for a specific molecular system. Typically it contains atom numbers, force constants and equilibrium dimensions. Usually one would start with a high level description of a molecular system (such as an amino acid sequence for a protein or the base sequence for a nucleic acid) and use a force field assembly program to generate a descriptor file. The file can be modified (with great care). It is possible to create a descriptor from scratch but this is tedious and error-prone. Unfortunately it may be necessary to do so as force field assembly programs exist only for reduced representation DNA and RNA (and these are specific representations out of many) and no general assembler yet exists.

The descriptor file consists of a file header line and any number of records each identified by a four character string. Each type of record should occur only once; a second and subsequent ocurrence will overwrite the previous record. Other than the file header the records can occur in any order. The header line (of the file and of each record) contains a keyword and one or more numbers; these fields may be separated by any number of white space characters but all the fields must fit on one line. Other fields of the records are treated as a single stream of numbers and they can be arranged in any manner. Be careful not to be fooled by your own formatting!

Text between the # character and the end of the line are ignored. Such comments can be placed in between records but not within a record.

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The descriptor file may occur in the binary or text forms. These pages describe the text form of the descriptor while the binary forms are required when running yammp. To interconvert between the two forms use des. This utility can also analyze a descriptor for common errors.

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In the following, descriptor file entries are typeset in teletype font, variables are italized and ellipses ... indicate more of the previous object. The following table contains a brief description of the various records that make up a descriptor: