Determination of fundamental properties of nitrogen from first principles. II. Interaction potential and spectroscopic properties of N$_2$

This work is the second part of the series devoted to first-principles determination of the fundamental properties of nitrogen.

In this part, we calculate the potential energy curve for the electronic ground state of N$_2$.

The potential is divided into three components: short-, medium-, and long-range, and a different computational protocol is applied to each component, based on a composite coupled cluster approach, genuine multireference methods, and asymptotic expansion of the interaction energy.

A particular focus is on the short-range part, as the accuracy of this component is critical from the point of view of metrological applications, including the temperature dependence of many properties of nitrogen gas.

Uncertainties of the theoretical data, originating both from basis set incompleteness and applied theoretical method, are rigorously analyzed.

The developed potential energy curve is used to calculate the spectroscopic parameters of the N$_2$ ground state and the results are compared with the available theoretical and experimental data.