Some remarks on the core resonance integral as used in the semi‐empirical π‐electron calculations

A simple semi‐theoretical expression for β, the core resonance integral as used in the semi‐empirical π‐èlectron calculations has been proposed. β's for some standard C C and C 0 lengths have been evaluated with the proposed formula and the calculated values are compared with those obtained by other semi‐theoretical and empirical methods. Electronic spectra of ethylene, formaldehyde, trans‐butadiene and benzene have been calculated by a Pariser‐Parr‐like semi‐empirical method with the values of β thus obtained. The calculated spectra are found to be in good agreement with the observed ones.