Few books are devoted to computational chemistry programming. If you would like to understand the code of Multiwfn, I suggest:
(1) Read a Fortran programming book of introductory level
(2) Read "Quantum Chemistry" 7ed by Levine to gain enough basic knowledge about quantum chemistry
(3) Read "Modern quantum chemistry" by Szabo, you will gain knowledge about algorithm and numerical details of quantum chemistry
(4) Read papers related to the wavefunction analysis in Multiwfn that you are interested inAfter reading the above materials, you will have enough ability and knowledge to understand the code of Multiwfn. The code of Multiwfn is really not difficult to read.
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