mopac: add page (#13122)

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: spageektti <git@spageektti.cc>

pages/linux/mopac.md

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+# mopac
+
+> MOPAC (Molecular Orbital PACkage) is a semiempirical quantum chemistry program based on Dewar and Thiel's NDDO approximation.
+> More information: <https://github.com/openmopac/mopac>.
+
+- Perform calculations according to an input file (`.mop`, `.dat`, and `.arc`):
+
+`mopac {{path/to/input_file}}`
+
+- Minimal working example with HF that writes to the current directory and streams the output file:
+
+`touch test.out; echo "PM7\n#comment\n\nH 0.95506 0.05781 -0.03133\nF 1.89426 0.05781 -0.03133" > test.mop; mopac test.mop & tail -f test.out`