Unusual RYTOEV value used in VASP

[xiancai]

Dear VASP developers,

Our group is trying to use VASP with i-pi, and found that the potential energy values output by i-pi differ slightly from those produced by VASP. We found that this discrepancy originates from the conversion factor RYTOEV used in VASP, i.e., 13.605826_q, which differs from the value reported by NIST (
https://physics.nist.gov/cgi-bin/cuu/Value?rydhcev) and other references such as the CRC Handbook of Chemistry and Physics, i.e., 13.605693.

We are a bit puzzled by this difference, as it results in a systematic shift in the energy values output by VASP. Could you please clarify the reason for using this particular value? Thank you very much!

Sincerely,
Tian Hua

[ferenc_karsai]

You are absolutely correct in your observation that VASP uses a Rydberg constant value (RYTOEV = 13.605826) that differs from the current NIST recommended value (13.605693). This is indeed intentional and stems from VASP's long development history spanning over 35 years.
When VASP was originally developed, the code was implemented using the best available physical constants at that time. Over the decades, improved experimental measurements have led to more precise determinations of fundamental constants, including the Rydberg constant. However, we have deliberately chosen not to update this value in VASP for several important reasons:

-) Reproducibility: Changing the conversion factor would make it impossible to reproduce calculations from the past 35+ years, which is crucial for scientific validation and comparison.

-) Consistency with existing literature: An enormous body of scientific publications and databases has been built using VASP with the current conversion factor. Updating it would create inconsistencies with this extensive body of work.

-) Backward compatibility: Many research groups rely on being able to reproduce and extend previous calculations, which would be compromised by such a fundamental change.