A new method for calculating internal conversion rate constants ( k&cmb.b.line; IC ), including anharmonic effects and using the Lagrangian multiplier technique, is proposed. The deuteration effect on k&cmb.b.line; IC is investigated for naphthalene, anthracene, free-base porphyrin ( H 2 P ) and tetraphenylporphyrin ( H 2 TPP ). The results show that anharmonic effects are important when calculating k&cmb.b.line; IC for transitions between electronic states that are energetically separated (Δ E ) by more than 20 000-25 000 cm −1 . Anharmonic effects are also important when Δ E < 20 000-25 000 cm −1 and when the accepting modes are X-H stretching vibrations with a frequency larger than 2000 cm −1 . The calculations show that there is mixing between the S 1 and S 2 states of naphthalene induced by non-adiabatic interactions. The non-adiabatic interaction matrix element between the S 1 and S 2 states is 250 cm −1 and 50 cm −1 for the normal and fully deuterated naphthalene structure and this difference significantly affects the estimated fluorescence quantum yield. Besides aromatic hydrocarbons H 2 P and H 2 TPP , the k&cmb.b.line; IC rate constant is also calculated for pyrometene (PM567) and tetraoxa8circulene (4B) with a detailed analysis of the effect of the vibrational anharmonicity. A new method for calculating internal conversion rate constants ( k IC ), including anharmonic effects and using the Lagrangian multiplier technique, is proposed.