A new method for calculating internal conversion rate constants (_IC), including anharmonic effects and using the Lagrangian multiplier technique, is proposed. The deuteration effect on _IC is investigated for naphthalene, anthracene, free-base porphyrin (H₂P) and tetraphenylporphyrin (H₂TPP). The results show that anharmonic effects are important when calculating _IC for transitions between electronic states that are energetically separated (ΔE) by more than 20 000–25 000 cm⁻¹. Anharmonic effects are also important when ΔE < 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 S₁ and S₂ states of naphthalene induced by non-adiabatic interactions. The non-adiabatic interaction matrix element between the S₁ and S₂ 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 H₂P and H₂TPP, the _IC rate constant is also calculated for pyrometene (PM567) and tetraoxa[8]circulene (4B) with a detailed analysis of the effect of the vibrational anharmonicity.