Uncertainty of High Temperature Heat Capacities: The Case Study of the NH Radical

Abstract

The difficulties in calculation of high temperature heat capacities and discrepancies between available data are discussed in the case of the NH molecule. The source of those inaccuracies are the underlining potential energy curves. The relevant partition functions and heat capacities are calculated with the classical Wigner–Kirkwood corrected method. In addition to the ground electronic state, also four excited states are taken into account. Two potential energy curves and experimental energy levels of the ground state are considered.

N–H collision integrals with study of repulsive interactions

“…It is interesting to compare discrepancy of various results of heat capacity and collision integrals. According to the study of NH molecule thermodynamics [20], at temperatures 5000-10 000 K the discrepancy of heat capacity based on various PECs is below 4% (for partition function even less) but the present study gives differences in σ 2 Ω (1,1) * and σ 2 Ω (2,2) * calculated on various PECs can exceed 10% at low temperatures. It shows that for calculating the collision integrals, the quality of PECs in all regions is more important than in the case of thermodynamic properties.…”

“…For the 3 Σ and 5 Σ state the ab initio PECs of Li and Zhu [18] were used and for the 3 Σ state also the curve of Koput [19], all in the Murrell-Sorbie functional form V(r) = −D e [1 + a 1 (r − r e ) + a 2 (r − r e ) 2 + a 3 (r − r e ) 3 ] exp[−a 1 (r − r e )]. In the previous study of NH radical thermodynamics it was concluded that for the 3 Σ state the Koput PEC is preferred over the one of Li and Zhu [20], that is why the Koput PEC will be used to calculate the final collision integrals (other curves are taken into account solely for the purpose of presenting the consequences of deviations from the optimal best known curve). TheHublurt-Hirschfelder(V(r…”

Electronic Free Energy Surface of the Nitrogen Dimer Using First-Principles Finite Temperature Electronic Structure Methods

“…The Helmholtz free energy is an important thermodynamic quantity that can be used to describe chemical processes. A part of this is the electronic free energy and consideration of this quantity is important in solid state materials, − spin-crossover systems, and in some cases, other molecular behavior. − In the case of solids it is well-known that the electronic free energy alone can be a significant contribution in the accurate description for the material. − On the other hand, the importance of electronic temperature in molecules has been demonstrated for specific situations such as spin-crossovers and at high temperatures. ,,, However, there is a need for more work to be done to fully understand the scope of how and when electronic temperature matters in molecules . One way to explore the scope is through ab initio electronic structure calculations.…”

“… 2 − 7 On the other hand, the importance of electronic temperature in molecules has been demonstrated for specific situations such as spin-crossovers 8 and at high temperatures. 11 , 13 , 17 , 19 However, there is a need for more work to be done to fully understand the scope of how and when electronic temperature matters in molecules. 13 One way to explore the scope is through ab initio electronic structure calculations.…”

New generalized potential energy function for diatomic systems

“…Diatomic potential energy functions are very useful in the predictions and rationalization of spectroscopic observations [3][4][5][11][12][13]. Such functions can also be used as building blocks in potential energy functions for small poly-atomic systems [14,15] and to calculate thermochemistry data [16]. More accurate representations are also used to explore the nature of interatomic interactions in ultracold regimes [9,10,17].…”