MOPAC Input Files

Using the AM1 water optimization as an example, the following is a typical MOPAC input file:
 

 AM1 PRECISE EF T=48H
 test3 AM1 optimization of water

  O    0.00000000  0    0.0000000  0    0.0000000  0    0    0    0
  H    1.00000000  1    0.0000000  0    0.0000000  0    1    0    0
  H    1.00000000  1  105.0000000  1    0.0000000  0    1    2    0

The first line contains all keywords relevant to the current calculation:
 

 AM1 PRECISE EF T=48H

AM1 defines the theoretical method used, PRECISE tightens up convergence criteria for all optimization processes (geometric or electronic) by a factor of 100 and EF calls the Eigenvector following algorithm for geometry optimization. T=48H sets the maximum runtime to 2 days beyond which MOPAC will shut down the calculation in an orderly manner. By default MOPAC performs a geometry optimization for the structure given in the input file. Thus there is no keyword to explicitly define a geometry optimization.

The second line of the input file is reserved for user comments (free format).

After one blank line the geometry of the system is given using an internal coordinate system, one center per line. Each line starts with the atomic symbol, followed by a bond distance, a bond angle and a dihedral angle. Whether these parameters are varaibles or constants during the geometry optimization is indicated by a trailing 0 (constant) or 1 (variable). At the end of each line the connectivity of the sytstem is specified through three integers. Using the third line as an example:
 

  H    1.00000000  1  105.0000000  1    0.0000000  0    1    2    0

we see that a hydrogen atom is connected to atom 1 with an intial distance of 1.0 angstroms and forms a bond angle with atoms 1 and 2 with an initial value of 105.0 degrees. Both parameters are marked as variables.