>>14320711You can, and they do with remarkable accuracy when possible. After all chemistry is "just" the study of interacting elections (mostly), so just apply quantum mechanics to systems of electrons and you're good to go.
The problem is that solving the n-electron quantum mechanical problem is generally exponentially hard in n. The equations are simply intractable for even modest size molecules. There are a number of approximations to reduce the problem to one of only polynomial complexity, and this is the entire field of quantum chemistry. But the approximations are typically less accurate for strongly interacting systems, so there is no general solution to the many-electron problem that achieves sufficiently small error.
>>14320914Relativistic effects are real and experimentally verified by the fine and hyperfine splitting of atomic spectra. Take your meds.
In quantum chemistry, again due to the inherent approximation error that practical algorithms have, it is rare to be concerned with such hyperfine details. But sometimes people are interested in attaining "stereoscopic precision", which just means orders of magnitude lower error that typical applications require. In this case, the molecular Hamiltonian is modified to include relativistic terms like spin-orbit coupling.
