first principles vs. fitting parameters

[alexander_s]

Perhaps one calls a model “ab initio” if it describes a wide range of phenomena and also depends on a small number of external fitting parameters. For example, solving Schroedinger equation for electronic structure is an ab initio approach, while using tight-binding models is not.

Classical and quantum mechanics, as well as Electromagnetism based on Maxwell’s equations, have their sets of “first principles”, and so can be regarded as ab initio models.

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