Example of data for ethylene

     Line   1 :     UHF PULAY MINDO3 VECTORS DENSITY LOCAL T=300 
     Line   2 :      EXAMPLE OF DATA FOR MOPAC
     Line   3 :        MINDO/3 UHF CLOSED-SHELL D2D ETHYLENE
     Line   4a:    C 
     Line   4b:    C    1.400118  1 
     Line   4c:    H    1.098326  1  123.572063  1 
     Line   4d:    H    1.098326  1  123.572063  1  180.000000  0   2  1  3
     Line   4e:    H    1.098326  1  123.572063  1   90.000000  0   1  2  3
     Line   4f:    H    1.098326  1  123.572063  1  270.000000  0   1  2  3
     Line   5 :

As can be seen, the first three lines are textual. The first line consists of keywords (here seven keywords are shown). These control the calculation. The next two lines are comments or titles. The user might want to put the name of the molecule and why it is being run on these two lines.

These three lines are obligatory. If no name or comment is wanted, leave blank lines. If no keywords are specified, leave a blank line. A common error is to have a blank line before the keyword line: this error is quite tricky to find, so be careful not to have four lines before the start of the geometric data (lines 4a-4f in the example). Whatever is decided, the three lines, blank or otherwise, are obligatory.

In the example given, one line of keywords and two of documentation are shown. By use of keywords, these defaults can be changed. Modifying keywords are +, &, and SETUP. These are defined in the KEYWORDS chapter. The following table illustrates the allowed combinations:

 
  Line 1      Line 2     Line 3  Line 4   Line 5  Setup used
 
  Keys        Text       Text   Z-matrix Z-matrix  not used 
  Keys +      Keys       Text    Text    Z-matrix  not used
  Keys +      Keys +     Keys    Text    Text      not used
  Keys &      Keys       Text   Z-matrix Z-matrix  not used
  Keys &      Keys &     Keys   Z-matrix Z-matrix  not used
  Keys SETUP  Text       Text   Z-matrix Z-matrix 1 or 2 lines used
  Keys +      Keys SETUP Text    Text    Z-matrix 1 line used
  Keys &      Keys SETUP Text   Z-matrix Z-matrix 1 line used

No other combinations are allowed.

The proposed use of the SETUP option is to allow a frequently used set of keywords to be defined by a single keyword. For example, if the default criteria are not suitable, SETUP might contain:

       " SCFCRT=1.D-8  SHIFT=30 ITRY=600 GNORM=0.02 ANALYT "
       "                                                   "

The order of usage of a keyword is:

Line 1 > Line 2 > Line 3.  
Line 1 > SETUP.  
Line 2 > SETUP.  
SETUP > built in default values.

The next set of lines defines the geometry. In the example, the numbers are all neatly lined up; this is not necessary, but does make it easier when looking for errors in the data. The geometry is defined in lines 4a to 4f; line 5 terminates both the geometry and the data-file. Any additional data, for example symmetry data, would follow line 5.

Summarizing, then, the structure for a MOPAC data-file is:

Line 1

Keywords. (See chapter 2 on definitions of keywords)

Line 2

Title of the calculation, e.g. the name of the molecule or ion.

Line 3

Other information describing the calculation.

Lines 4

Internal or cartesian coordinates (See chapter on specification of geometry)

Line 5

Blank line to terminate the geometry definition.

Other layouts for data-files involve additions to the simple layout. These additions occur at the end of the data-file, after line 5. The three most common additions are:

• Symmetry data: This follows the geometric data, and is ended by a blank line.
• Reaction path: After all geometry and symmetry data (if any) are read in, points on the reaction coordinate are defined.
• Saddle data: A complete second geometry is input. The second geometry follows the first geometry and symmetry data (if any).