Sample directory: adf/e_Freq_NH3/
Computation of frequencies by Cartesian displacements. The assumed equilibrium input structure is given in internal coordinates. A dummy atom is used for a convenient definition of the Z-matrix such that it reflects the pointgroup symmetry C(3v).
$ADFBIN/adf <<eor title NH3 frequencies define rNH=1.02 theta=112 phi=120 end atoms Z-matrix XX 0 0 0 N 1 0 0 1.0 H 2 1 0 rNH theta H 2 1 3 rNH theta phi H 2 1 4 rNH theta phi end Basis Type TZP Core Small End geometry optim cartesian frequencies end thermo T=300,400 integration 5.0 end input eor
The symmetry is determined automatically by the program as C(3v), from the input coordinates. In a Frequencies calculation the symmetry (specified on input or computed internally) is used for analysis and in some cases to speed up the calculation.
The equilibrium coordinate values are supplied as identifiers that are associated with values in the define block.
Unlike using the geovar key, applying the define key does not mean anything in the sense that the various coordinates that refer to the same identifier would be forced to remain equal; it is just a way to display (to the human reader) symmetry in the equilibrium values, to avoid typing errors and to allow an easy adjustment of starting coordinates for another calculation.
Since the atomic coordinates are input in Z-matrix format, the program would by default carry out displacements in internal coordinates to scan the energy surface and hence compute force constants and frequencies. This is overriden by specifying in the geometry block optim cartesian: carry out cartesian displacements.
The key thermo addresses the thermodynamical analysis (only available in a Frequencies calculation, otherwise ignored). The specification 'T=300,400' means that the thermodynamic properties are printed for the temperature range 300-400K, in steps of 10K (default) and for a pressure of 1.0 atmosphere (default).
Frequencies calculations suffer easily from numerical inaccuracies. Therefore, the default numerical integration precision in a Frequencies calculation is much higher than in an ordinary single-point or minimization run. Here we specify the INTEGRATION level to be 5.0 (quite high, but the default for Frequencies is even 6.0).
