Calculating frequencies and thermochemistry is critically important in computational chemistry.
Calculating a molecule's vibrational frequencies involves computing the second derivatives of the energy and then decomposing these into molecular vibrations and associated force constants to generate a vibrational spectrum. This task is very important in computational chemistry, as it is used to validate that all structures are true ground or transition states. (Ground states possess zero imaginary frequencies, while transition states possess a single imaginary frequency.) Furthermore, the imaginary frequency associated with a transition state can be visualized to show which atoms are reacting, thus showing whether the transition state is real or spurious.
Rowan uses state-of-the-art techniques to ensure that vibrational frequencies are computed as quickly and accurately as possible. Our software automatically projects out rotational and translational degrees of freedom to ensure that the generated vibrational modes aren't contaminated by any additional molecular movements. Rowan automatically checks the magnitude of all rotational and vibrational modes to ensure that this process is reliable, and alerts the user if excessive frequencies are detected—often a sign of errors in the calculation!
Our web interface makes it easy to visualize the motion associated with each vibrational frequency, and automatically alerts the user if the ground or transition state contains extraneous imaginary frequencies that might invalidate the result.
Using Rowan's machine-learned interatomic potentials, vibrational frequencies can be computed much faster than with conventional methods. Machine learning-based methods can compute the matrix of second derivatives (needed for frequencies) virtually for free, whereas this computation typically requires significant effort with physics-based methods. With Rowan, you can calculate accurate vibrational frequencies, even for large structures, in only seconds!
Using the vibrational modes that a frequency calculation generates, Rowan can compute approximations for the vibrational, rotational, and translational components of entropy, thus permitting computation of thermochemical properties like zero-point energy, Gibbs free energy, and enthalpy. All values are displayed in an easy-to-read table, and can be copied to the clipboard for further analysis.
To prevent excessive contributions from small vibrational modes under the rigid-rotor-harmonic-oscillator approximation, Rowan employs the Cramer–Truhlar quasiharmonic correction for all thermochemical calculations, ensuring accurate and reliable free energy values.
