In order to understand the nature of the driving force for distorting the non-planar molecule benzylideneaniline (NBA) away from its planar geometry, two energy effects, the vertical resonance energy DE^V(θ) and the σ-π orbital interaction energy ΔE^σπ(θ), were calculated with the DFT method, and then partitioned into their π and σ parts, denoted as DE^V-π(θ) and DE^V-σ(θ), DE^(σπ)-π(θ) and DE^(σπ)-σ(θ) respectively. DE^V(θ) is always destabilizing, and has a tendency to distort NBA molecule away from its planar geometry as far as possible. Similarly, DE^σπ(θ) is also destabilizing, however, it is most destabilizing at the θ＝90° geometry. NBA molecule would prefer the θ＝90° geometry if there were no interaction between the σ and p systems. The fact dE^T(θ)/dθ＝0 (total energy) around θ＝40° geometry, is a compromise between the various orbital interactions including π-π, σ-π interactions.

Key words: energy separation, density functional theory, vertical resonance energy, σ-π orbital interaction energy, benzylideneaniline