Specification of keywords

All control data are entered in the form of keywords, which form the first line of a data-file. A description of what each keyword does is given in Section 2.3. The order in which keywords appear is not important although they must be separated by a space. Some keywords can be abbreviated, allowed abbreviations are noted in Section 2.3 (for example 1ELECTRON can be entered as 1ELECT). However the full keyword is preferred in order to more clearly document the calculation and to obviate the possibility that an abbreviated keyword might not be recognized. If there is insufficient space in the first line for all the keywords needed, then consider abbreviating the longer words. One type of keyword, those with an equal sign, such as, BAR=0.05, may not be abbreviated, and the full word needs to be supplied.

Most keywords which involve an equal sign, such as SCFCRT=1.D-12 can, at the user's discretion, be written with spaces before and after the equal sign. Thus all permutations of SCFCRT=1.D-12, such as SCFCRT =1.D-12, SCFCRT = 1.D-12, SCFCRT= 1.D-12, SCFCRT = 1.D-12, etc. are allowed. Exceptions to this are T=, T-PRIORITY=, H-PRIORITY=, X-PRIORITY=, IRC=, DRC= and TRANS=. ` T=' cannot be abbreviated to ` T ' as many keywords start or end with a `T'; for the other keywords the associated abbreviated keywords have specific meanings.

If two keywords which are incompatible, like UHF and C.I., are supplied, or a keyword which is incompatible with the species supplied, for instance TRIPLET and a methyl radical, then error trapping will normally occur, and an error message will be printed. This usually takes an insignificant time, so data are quickly checked for obvious errors.