Why do all Quantum Chemistry programs use "Atomic Units"?

    Why do all quantum chemistry programs use atomic units, not calories or joules, in the calculation? 

       If you have been experiencing quantum calculation (either computational chemistry or physics), you might suspect that the physical quantity printed in the output file is not the normal one that you are familiar with. 

       This is because the program developer wants to speed their code up, the quantum program is not good at calculating decimals. An atomic unit is a central unit in which most of the necessary physical quantities, such as the mass of the electron (m_e), time (t), the atomic unit of energy (Hartree), and electron charge (e) are equal to 1! That is 1.000000 . . . 

It means that using "atomic atomic" can make the program more accurate which comes with numerical integration. Again, when those parameters are set to 1, the computational complexity is greatly reduced! This would make the program run faster and produce reliable results at the same time.

The table below is the unit conversion between the atomic unit and the S.I. unit.

From the table, the relation of all physical quantities is 

L = m_e = e = Hartree = ħ = 1. 

So, the program will convert atomic units to S.I. units when the calculation is finished.

The following post is FORTRAN 77 code which can be used for energy conversion. Energy Units and Conversion using FORTRAN

Rangsiman Ketkaew