Foundations of quantum chemistry#
Part I showed you how to drive qc-rs. This part explains what it is actually computing. It is the theoretical heart of the manual — written to be read like a short textbook, from the physical problem down to the equations a program like qc-rs solves. You do not need it to run a calculation, but you do need it to understand your results, choose sensible methods, and know when to trust a number.
We build the story in four steps, each its own chapter:
The many-electron problem & Born–Oppenheimer — the molecular Schrödinger equation, why it cannot be solved exactly, and the first great simplification (freezing the nuclei).
Variational principle, LCAO & basis sets — how we turn an impossible differential equation into a finite matrix problem: expand the unknown orbitals in a basis and minimize the energy.
Hartree–Fock — the foundational approximation: one Slater determinant, the self-consistent-field equations, and what it gets right and wrong.
Density functional theory & Kohn–Sham — recasting the problem in terms of the electron density, and the workhorse of modern quantum chemistry.
Note
How to read this part
Each chapter states the theory with its equations, then connects it back to what you do in qc-rs (the
ao= basis, ref="r", xc="b3lyp", and so on). Read it straight through the first time; later, come back
to a single chapter when a term in the User guide needs unpacking. We use
atomic units throughout — introduced in the next chapter.
Ready? Start with the many-electron problem.