# Pair potential with submillikelvin uncertainties and nonadiabatic treatment of the halo state of helium dimer

## TL;DR

This paper presents a highly precise helium pair potential with submillikelvin uncertainties and introduces a nonadiabatic approach to accurately describe the helium dimer's halo state, improving theoretical predictions.

## Contribution

The study significantly enhances the helium pair potential accuracy and applies a novel nonadiabatic method to predict the halo state's properties.

## Key findings

- Pair potential uncertainties below 1 millikelvin in the well region.
- Predicted halo state binding energy of 138.9(5) neV.
- Agreement with experimental binding energy within uncertainties.

## Abstract

The pair potential for helium has been computed with accuracy improved by an order of magnitude relative to the best previous determination. For the well region, its uncertainties are now below 1 millikelvin. The main improvement is due to the use of explicitly correlated wave functions at the nonrelativistic Born-Oppenheimer (BO) level of theory. The diagonal BO and the relativistic corrections were obtained from large full configuration interaction calculations. The nonadiabatic perturbation theory was used to predict the properties of the halo state of helium dimer. Its binding energy and the average value of interatomic distance are found to be 138.9(5) neV and 47.13(8) {\AA}. The binding energy agrees with its first experimental determination of 151.9(13.3) neV [Zeller et al., PNAS 113, 14651 (2016)].