Exploring NH3–NH3 interactions: A comparative study of force field and CCSD(T)/QZVPP calculations for thermodynamic analysis and second virial coefficient in gas-phase chemistry and atmospheric science

In this study, we investigate the quadrupole moment of ammonia (NH 3 ) using the coupled cluster method with single, double, and perturbative triple excitations [CCSD(T)] and the quadruple-zeta valence plus polarization (QZVPP) basis set. The calculated values for the quadrupole moment, polarizability , and entropy of NH 3 (−2.44 a.u, 2.113 Å 3 , 192.69 JK −1 mol −1 ) are in excellent agreement with the corresponding experimental values (−2.45 a.u, 2.109 Å 3 , 192.77 JK −1 mol −1 ) respectively. The vibrational mode of frequencies of ammonia is close to calculated values. Additionally, we explore the application of ILJP parameters (ILJPP) to determine the SVC B of NH 3 . By employing the ILJP limitations, we obtain calculated values of B that exhibit a remarkable agreement with the experimental values. This result highlights the accurateness and reliability of the ILJP in describing the intermolecular interactions of NH 3 dimer. Our findings demonstrate the capability of the CCSD(T)/QZVPP method to accurately determine the quadrupole moment of NH 3 , validating its agreement with experimental values. Furthermore, the successful implementation of ILJPP to calculate the SVC emphasizes the effectiveness of this approach in capturing the thermodynamic properties of NH 3 . This research contributes to a deeper understanding of NH 3 's molecular properties and facilitates its application in various scientific and technological domains. In gas-phase chemistry , knowledge of ammonia-ammonia interactions is essential for predicting reaction rates, exploring molecular dynamics , and understanding gas-phase equilibria involving ammonia. Furthermore, in atmospheric science , studying NH 3 –NH 3 interactions can contribute to our understanding of ammonia's role in air pollution, aerosol formation , and acid deposition . A set of 1500 random conformations of NH 3 –NH 3 molecules were subjected to QZVPP calculations to determine their interaction energies. To accurately represent NH 3 –NH 3 interactions, an Improved Lennard-Jones potential (ILJP) energy surface was employed to fit these energies. The validity of the resulting conformations was verified, and their close correspondence with experimental observations enhances the reliability of the approach. By utilizing the fitted potential, the determination of the second virial coefficient (SVC) B exhibited excellent arrangement with experimental values, showcasing the highest precision of the method. • A set of 1500 random conformations of NH 3 –NH 3 molecules were chosen to determine interaction energies. •Improved Lennard-Jones potential (ILJP) energy surface was employed to fit these energies. •Studying NH 3 –NH 3 interactions can contribute to our understanding of ammonia's role in air pollution