The presence of non-bonding molecular orbitals diminishes the HMO total pi-electron energy. A method is elaborated to quantitatively assess this effect. The effect is found to be much smaller than earlier anticipated. Contrary to earlier claims, it depends on the size of the conjugated system and, as a good approximation, decreases as the pth power of the number of vertices of the molecular graph; p approximate to 0.95. The effect is almost insensitive to other structural features present in the conjugated system. (C) 2003 Elsevier B.V. All rights reserved.