# Interaction between two excited hydrogen atoms

hi all.

i am trying to calculate the interaction between two excited hydrogen atoms, using degenerate perturbation theory. but comes up a few problems, any helps will be greatly appreciated.

the perturbation has the form:

H=e3/R3(XaXb+YaYb-2ZaZb)
where the R is the distancee between two atoms and a , b subscripts donates atom a and b respectively.

i need to calculate the matrix elements have the form:
(<ψaψb H ψaψb>

first thing is how to construct the total wave functions of the two atoms system? they add together or times together? and why? what is the physics meaning of:
ψtotalaψb
ψtotalab

the second is, if using the exzact form, and ignor the radial funtion, is it possible to evaluate the matrix element? i.e
for one atom in its 2 0 0 state and the other in its 2 1 0 state. then wavefunctions equal to theirs spherical harrmonics which are:

2 0 0 state = 1/(2* square root pi )
2 1 0 state = square root 3 /(2* square root pi) * cosθ

couples to one atom in 2 1 0 state and the other in 2 1 1 state

2 1 1 state = squre root (3)/2 * 1/(squre root (2* pi)) * sinθ*ei∅

the first term yields:

∫∫∫2 0 0 state * Xa * 2 1 0 state dr sinθ dθ d∅ *∫∫∫2 1 0 state * Xb * 2 1 1 state dr sinθ dθ d∅

since x= rsinθcos∅ and we have ignored the radial part, how can I evaluate dr ?
above all, am I right for constructing the matrix element like the above way?

Apologise for such a messy question, it is my first time to put equations up here, and I havent figured out how to construct a complex equation.