diff --git a/tldr/linux/mopac b/tldr/linux/mopac
new file mode 100644
index 00000000..c9baff7e
--- /dev/null
+++ b/tldr/linux/mopac
@@ -0,0 +1,21 @@
+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# 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
+#comment
+
+H 0.95506 0.05781 -0.03133
+F 1.89426 0.05781 -0.03133" > test.mop; mopac test.mop & tail -f test.out`
diff --git a/tldr/sort b/tldr/sort
index 5499fd8e..880f72bc 100644
--- a/tldr/sort
+++ b/tldr/sort
@@ -28,6 +28,10 @@ source: https://github.com/tldr-pages/tldr.git
 
 `sort --field-separator={{:}} --key={{3n}} {{/etc/passwd}}`
 
+- As above, but when items in the 3rd field are equal, sort by the 4th field by numbers with exponents:
+
+`sort -t {{:}} -k {{3,3n}} -k {{4,4g}} {{/etc/passwd}}`
+
 - Sort a file preserving only unique lines:
 
 `sort --unique {{path/to/file}}`
@@ -35,7 +39,3 @@ source: https://github.com/tldr-pages/tldr.git
 - Sort a file, printing the output to the specified output file (can be used to sort a file in-place):
 
 `sort --output={{path/to/file}} {{path/to/file}}`
-
-- Sort numbers with exponents:
-
-`sort --general-numeric-sort {{path/to/file}}`