Example of Data for Ethylene

Ethylene is a simple organic molecule, of formula C₂H_4,shown in the figure below.

Figure 1: Ethylene

For this calculation, the bond-lengths and angles can be assumed. Unless instructed not to do so, MOPAC will automatically optimize the geometry. For this system, the data set is as shown in Figure 2.

Figure 2: Data set for Ethylene

Line  1: VECTORS LOCAL PI PM6
Line  2: EXAMPLE OF MOPAC DATA-SET FOR ETHYLENE
Line  3: The only geometric constraint is that the system must be planar
Line  4: C
Line  5: C   1.3 1
Line  6: H   1.1 1   120 1   0 0     2  1
Line  7: H   1.1 1   120 1   0 0     1  2  3
Line  8: H   1.1 1   120 1 180 0     1  2  3
Line  9: H   1.1 1   120 1   0 0     2  1  5
Line 10:

As can be seen, the first three lines are textual. The first line consists of keywords (here four 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.

Lines one through three 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 difficult to find, so be careful not to have four lines before the start of the geometric data (lines 4-9 in the example). Whatever is decided, the three lines, blank or otherwise, are obligatory.

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 4 to 9; line 10 terminates both the geometry and the data-file. Any additional data, such as symmetry data, would follow line 10.

At any point in the data set, one or more comment lines can be introduced. These are identified by the presence of an asterisk (*) as the first character on the line.