Different zeta(sc) and zeta(td) values output by i-pi

[Xu Heseri]

Dear i-pi developers,

    I am trying using i-pi (version 3.0) to perform PIMD simulations to derive isotope fractionation factors. I have set the simulation to output the results from both the direct scaled-coordinate estimator (isotope_zetasc) and the direct thermodynamic estimator (isotope_zetatd) (please see the attached input.xml file).

    The average values of <exp(-beta*sc)> are 6.23 for H and 1.05 for O, respectively, which appear reasonable and validate the simulation (please see the attached simulation.SC file). However, as shown in the attached simulation.TD file, the average values of <exp(-beta*spr)> are extremely strange, i.e., 5.52E-14 for H and 4.33E-3 for O, respectively.

    To my understanding, for the same element in one simulation, these two values should be the same. I am wondering whether there might be an issue with my setup. If not, is there an underlying relation between these two values that could explain the discrepancy? Thank you very much for your time and support!

Cheers,

Xu

[Venkat Kapil]

Hi Xu, 

Not an expert in isotope fractionation ratios, but there seems to be a bug. This part of the code hasn't been used much. You are welcome to open a PR and contact the authors in the paper. 

Kind regards,
Venkat

[Xu Heseri]

Dear Prof. Kapil,

Thank you very much for your reply! Following your suggestion, I have opened an issue on the i-Pi github page, and hope to receive help from potential experts.

Sincerely,
Xu