Convert energy into Hartree energy in emacs

There is a neat program in emacs which is called (GNU Emacs Calc Manual). Up to now in order to do energy conversion I always wrote small scripts in whatever language to do this.

Now I discovered that you do these conversions also in emacs, you can do M-x calc, insert an algebraic form typing ' followed by 1.5 ev, which inputs 1.5 electron volt into the stack of the calculator.

Now you can change the units of this, by typing u c. You can change it to Kelvin for instance by typing Ken as the new units. Quantum chemists and physicists use often the units of Hartree atomic units - Wikipedia which is quite convenient for atomic systems.

There is no Hartree units in emacs calc, but you can write a simple function to define units,

(defun blog/define-unit (symbol def description)
  (setq math-additional-units
        (cons (list symbol def description)
              math-additional-units)
        math-units-table nil)
  (calc-invalidate-units-table))

(blog/define-unit 'ha "27.21139664130791 ev" "Hartree atomic units energy")

Now you can convert from eV to Ha from you beloved emacs!