A newcomer to density functional theory calculations is exposed to jargon which sounds rather non-technical at first. For instance, 'charge density' is something that is actually a term in the formalism, whereas terms like 'first principles' seem adjectival. Please help me understand what they mean and their relevance as to why are they associated with the theory almost everywhere.
First Principles mean starting directly at the level of established science and not making assumptions such as any empirical models or parameter fitting.
With respect to DFT, EMF (Electromagnetic force) is a very strong force governing nucleus and electrons (referring to a single atom).
With that, atoms, molecules, macro molecules and materials are built up. Whatever happens in materials (be it physical deformation or its fundamental properties) comes from this basic interaction.
The law that governs these small particles and their interaction is Quantum Mechanics. The calculation methods (DFT) based on QM do not use fitting parameters from experiments or assume anything. They are solely dependent on basic laws of physics (not Newton's laws).
Thus they are named as First Principle (ab initio) Methods.
Just to give an idea: The only information these calculation methods need is the Atomic Number of the atoms involved.