How to get convergence error messages or max gradient in lme4

How could I get max gradient values after doing random effects models with lme4? By default, it is shown if it is more than 0.002, but I want to make an original function as below, which needs a returned value of max gradient, whether the value is more than 0.002 or not.

1. Doing the original model

MM<- lmer(Y ~ 1 + X + (1 |cluster_ID ), data=data)
summary(MM)

2. If the above original model have the max gradient more than 0.002, then do the below process.

MM_par <- getME(MM, c("theta", "fixef"))
RS_MM <- update(MM, start=MM_par, control=lmerControl(optCtrl = list(maxeval=1e4)))
summary(RS_MM)

Example:

library(lme4)
fm1 <- lmer(Reaction~Days+(Days|Subject), sleepstudy)

Extract the @optinfo slot and see what's there:

names(fm1@optinfo)

Check the $derivs element:

fm1@optinfo$derivs

Here's the gradient:

max(abs(fm1@optinfo$derivs$gradient))

But we want the scaled gradient (thanks @RobertLong for the reminder)

dd <- fm1@optinfo$derivs
sc_grad <- with(dd, solve(Hessian, gradient))
max(abs(sc_grad))

notes

• extracting slots directly via @ is not 100% guaranteed to be future-proof (although lme4 is pretty stable)
• increasing the maximum number of evaluations won't actually change the max gradient. You might consider tightening/decreasing the convergence tolerance instead (see ?lme4::convergence)