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A TITLE is supplied. This title is printed in the output header. It is also written to any result files from the calculation and will be printed out when such a file is attached to another calculation, for instance as a fragment file. In addition, ADF constructs a 'jobidentification' string that contains the ADF release number and the date and time. The jobidentification is also printed in the output header and dumped on any result files.
The atomic positions are given with the key ATOMS. Bond lengths are in Angstrom and angles in degrees. The key GEOMETRY assigns numerical starting values to the two variables. We could also have written numerical values directly in the ATOMS block. The structure used here implies that the two HO bonds are equal and must remain equal: they are associated with the same variable; this constraint would not have applied if numerical data had been put in the ATOMS section, although the symmetry would have kept them equal anyway.
The key GEOMETRY must be supplied to let the program do an optimization: otherwise a single point calculation would be carried out. The GEOMETRY data block is empty here, meaning that no default values are reset for the options that are controlled with this key.
No symmetry is specified by a Schönfliess type symbol (key SYMMETRY). The program will use the true symmetry of the nuclear frame (accounting for any fields, if present). In this case that is C(2v). If such symmetry would not be acceptable for ADF (not all pointgroups are supported!) or when you want to run in a lower symmetry, the symmetry to be used must be specified.
The fragment files are defined under FRAGMENTS. In this case - as well as in most other samples - the fragment files reside in the local directory since they were created there in the same job. If they would have been located elsewhere you could specify a full path for each of the files, or alternatively (if all fragmentfiles are in one single directory) write the directory after the keyword FRAGMENTS (on the same line).
The precision of numerical integration, to evaluate Hamiltonian matrix elements etc., is not specified and attains therefore the default value (4.0 in an optimization run).