Very low acceptance for SU(2) 1 adjoint flavour RHMC about grid

[edbennett]

Two more things that I have checked:

• @LupoA pointed out that there is a normalisation factor of HMC_MOMENTUM_DENOMINATOR that is by default set to 2, while in HiRep this factor is not included. Setting this to 1 (via removing the #define CPS_MD_TIME in Grid/qcd/action/gauge/GaugeImplTypes.h and Grid/qcd/action/scalar/ScalarImpl.h) does not remove the discrepancy.
• HiRep multiplies the step size by beta / NG, which as far as I can see isn't done in Grid. Removing this factor further exacerbates the discrepancy.

[edbennett]

Some more testing shows that with a thermalised configuration (the same one for both codes), and controlling for all the factors above, the acceptances match much more closely between HiRep and Grid. Additionally, setting --Thermalizations to a number larger than zero (I've been using 20) will immediately overcome this initial barrier and allow the acceptance to stabilise at the same parameters as work for HiRep (which does not do this thermalisation step, as far as I am aware). This raises three possibilities that I can see:

• HiRep does some thermalisation that I'm not aware of (although I have searched and haven't found evidence of this)
• Grid's integrator behaves differently on very far-from-equilibrium configurations
• Grid initialises a hot start differently from HiRep