realQM is a physical model in terms of classical continuum mechanics of an atom or ion consisting of a pointlike kernel of positive charge surrounded by electrons of negative unit charge, as a *free boundary problem* for a system of distributed non-overlapping electron unit charge densities in 3d Euclidean space . For a neutral atom , while for an ion with positive charge and for an ion with negative charge , as an atom which has lost or gained electrons. The chemical properties of an atom is largely determined by the energy required to remove an electron (ionisation energy) and the energy released by capturing an electron (electron affinity), as prime tasks for an atom model.

The electron charge distribution of the ground state of an atom/ion is characterised by minimising a total energy as the sum over electrons of kernel potential energy, inter-electron potential energy and so-called kinetic energy as a measure of charge compression. Each electron minimises its contribution to the total energy under a free boundary condition of continuity of charge density.

realQM is a many-electron model as a collection of one-electron models over a partition in space coupled by electron potentials and satisfying a free boundary condition of charge continuity. As such realQM combines simplicity, generality and physicality.

In mathematical terms realQM starts from a wave function Ansatz

- (1)

as a sum of real-valued electron wave functions depending on a common Euclidean 3d space coordinate and having non-overlapping spatial supports with boundaries for , together filling .

We assume that for ,where is the set of real-valued functions defined on the domain in which are square integrable along with first derivatives.

We ask the electron wave functions to statisfy the normalization condition

- (2)

attributing unit charge to each electron with representing the charge density of electron .

We consider as the union of for to be a *free boundary* to be determined along with the wave function and as a free boundary condition, we shall ask to be continuous across $\Gamma$, that is that and agree on . We express satisfaction of the free boundary condition of continuity by asking that .

We start considering real-valued wave functions depending on the space coordinate , and later extend to complex-valued wave functions with time dependency. We seek the *neutral ground state* of the atom with as a real-valued function of the form (1) satísfying (2), which with the varying freely over minimizes the *total energy:*

- (3)

as the sum of *kinetic energy* (with Planck’s constant normalized to 1):

*attractive kernel potential energy*:

and *repulsive electronic potential energy:*

where is the potential generated by a pointlike positive kernel of charge and is the potential generated by electron defined by

- .

We see that the total energy of an electronic configuration defined by the wave function has a negative contribution from Coulombic attractive kernel potential energy , a positive contribution from Coulombic repulsive electronic potential energy without self-repulsion, and a positive contribution from as a measure of concentration of electron charge.

We see that

and understand that expresses that an electron does not interact with itself and that the factor 2 accounts for the double presence in the sum over all .

A minimising wave function satisfies the following system of one-electron Schrödinger equations expressing vanishing of the gradient of with respect to free variation of over under the side condition :

- (4)

for , where acts as a Lagrange multiplier for .

We observe the presence of the homogeneous Neumann condition

- ,

as a variationally imposed condition reflecting free variation of in . Further, the factor 2 in reflects the presence of in the equations for with through the potential .

We can thus formulate the effective boundary condition to be satisfied on the free boundary as follows:

- ,

with normal to . The free boundary thus carries both a homogeneous Neumann condition and a Dirichlet condition asking continuity which makes a connection to what is referred to as a *Bernoulli free boundary problem* for the Laplacian in a domain with a combined Neumann and Dirichlet condition on a part of the boundary.

We observe that the total energy of a minimising with eigenvalues is not given as because of the factor 2 of the electronic potential in (4).