# How to model large atoms

by VVS
Tags: atoms, model
 P: 67 Hi! One can easily analyze the Hydrogen Atom since it is a two body problem. But how do you apply Quantum Theory to model atoms (such as iron) which are much larger and predict their behaviour in an environment? My guess is that you use statistical mechanics, but I only just started a course and it is basically limited to heat. thank you VVS
 Mentor P: 11,928 The first approach is still a two-body problem. Afterwards, interactions between the electrons can be taken into account. To describe the state of the electrons and bonds in a material, this is pure quantum mechanics. If you want to describe things like heat, you don't have to care about those details, you take the "output" of quantum mechanics (crystal structure, energy bands and so on) and apply statistical mechanics to it.
 P: 67 Hi mfb! Thanks for your answer. Basically I want to evaluate the effect of electric fields, magnetic fields and magnetic vector potentials on the properties of Iron in haemoglobin. How do I go about this?
 Mentor P: 11,928 How to model large atoms I guess that will need some protein folding software if you expect effects - the fields influence the whole thing, not just a single small atom inside.
 P: 5 By the way, I think we analyze the Hydrogen atom by quantum mechanics is a one body problem, because we assume the nuclear is fixed, and it just provide a potential to the atom system, but in the real physics we should also use a wave-function to describe the nuclear
 Mentor P: 11,928 Usually the two-body problem is reduced to a 1-body problem with a reduced mass, so both electron and nucleus are taken into account. The other degrees of freedom of the two-body system correspond to a total motion of the atom.
 P: 430 OP, real atoms (and molecules) are handled with quantum chemistry software. Such programs (e.g., Molpro, Orca) can solve the many-body Schrödinger equations in various approximations to determine the quantitative behavior of the electrons, including their response to external fields. For Iron atoms, for example, you would employ approximations like Hartree-Fock and CCSD(T) (a coupled cluster method) or Multi-configuration self consistent field (MCSCF) and mutli-reference configuration interaction (MRCI), depending on the application. Understanding and using such approximations (correctly) is not easy, and normally requires some background reading in many-body quantum mechanics and quantum chemistry.

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