Write the important conditions required for the linear combination of atomic orbitals to form molecular orbitals.

(N/A) The linear combination of atomic orbitals $(LCAO)$ to form molecular orbitals takes place only if the following conditions are satisfied:
$1$. The combining atomic orbitals must have the same or nearly the same energy. For example,a $1s$ orbital can combine with another $1s$ orbital but not with a $2s$ orbital because the energy of the $2s$ orbital is significantly higher than that of the $1s$ orbital.
$2$. The combining atomic orbitals must have the same symmetry about the molecular axis. By convention,the $z$-axis is taken as the molecular axis. Atomic orbitals with similar energy will not combine if they do not possess the same symmetry. For example,a $2p_z$ orbital of one atom can combine with a $2p_z$ orbital of another atom,but not with $2p_x$ or $2p_y$ orbitals due to their different symmetries.
$3$. The combining atomic orbitals must overlap to the maximum extent. The greater the extent of overlap,the greater will be the electron density between the nuclei of the molecular orbital.