# Molecular orbitals and “generalized” aufbau principle for heteronuclear molecular configurations

Is there any general ordering or rule for heteronuclear molecules like $\ce{AX_2}$, $\ce{AX_{n}}$, and their molecular bonding/antibonding configurations ($\sigma/\pi$ orbitals) accordingly to some order in the Molecular Orbital Theory (MOT)?

Do the molecular orbital configurations and their order depend on the scheme and the linear combination of atoms (homo and heteronuclear, di and polyatomic) or is there some "standard ordering" like the aufbau principle $\ce{1s~2s~2p~3s~3p~4s~3d~4p~5s~\dots}$ for atomic orbitals in atoms? Or are molecular configurations a totally dependent on the molecule type and geometry? That is, I want to know if there is some (general) rule for molecular orbitals (ordering and energies) just like the aufbau principle for atomic orbitals.

## 1 Answer

Do the molecular orbital configurations and their order depends on the scheme and the linear combination of atoms (homo and heteronuclear, di and poliatomic)

Not only orbitals arising depends on atom configuration, but also on atomic orbitals available (s,p,d) and their ordering depends on geometry (i.e. size) and relative energy of atomic orbitals available. Unfortunately, there is no simple rule here.

Lucky enough, there is some amount of free computational packages you can use to calculate and then visualize molecular orbitals.

• I use Linux. Could you recommend me one? I am interested in the trial of some noble gas compounds...I would be happy if you could share or tell me how to find those packages. – riemannium Jul 7 '14 at 14:56
• afaik, gamess is free for personal use, though requires a registration. AFAIK, recent versions of avogadro are capable of visualizing MOs from gamess output. – permeakra Jul 7 '14 at 15:20