I am running a well-tempered metadynamics with an attempt to do it with multiple walkers. Unfortunately it fails at the moment first walker tries to deposit first hill. As a test case on my local machine I run only one walker (I suppose it's a realistic case, since it's technically possible that some walker will start way ahead of others), other walkers just stay as empty directories with all files. I am using set-up with replicas being independent jobs.
As I see in the log file, all necessary things were read:
colvars: Metadynamics bias "mtd": accessing replica "1".
colvars: Metadynamics bias "mtd": accessing replica "2".
colvars: Metadynamics bias "mtd": accessing replica "3".
colvars: Metadynamics bias "mtd": accessing replica "4".
colvars: Metadynamics bias "mtd": accessing replica "5".
colvars: Metadynamics bias "mtd": accessing replica "6".
colvars: Metadynamics bias "mtd": accessing replica "7".
colvars: Metadynamics bias "mtd": replica "1" has supplied a new state file, "walk_1/md.colvars.mtd.1.state".
colvars: Metadynamics bias "mtd": replica "2" has supplied a new state file, "walk_2/md.colvars.mtd.2.state".
colvars: Metadynamics bias "mtd": replica "3" has supplied a new state file, "walk_3/md.colvars.mtd.3.state".
colvars: Metadynamics bias "mtd": replica "4" has supplied a new state file, "walk_4/md.colvars.mtd.4.state".
colvars: Metadynamics bias "mtd": replica "5" has supplied a new state file, "walk_5/md.colvars.mtd.5.state".
colvars: Metadynamics bias "mtd": replica "6" has supplied a new state file, "walk_6/md.colvars.mtd.6.state".
colvars: Metadynamics bias "mtd": replica "7" has supplied a new state file, "walk_7/md.colvars.mtd.7.state".
colvars: Metadynamics bias "mtd": reading the state of replica "1" from file "walk_1/md.colvars.mtd.1.state".
colvars: successfully read the biasing potential and its gradients from grids.
colvars: successfully read 0 explicit hills from state.
colvars: Restarted metadynamics bias "mtd" with step number 0.
colvars: Metadynamics bias "mtd": reading the state of replica "2" from file "walk_2/md.colvars.mtd.2.state".
colvars: successfully read the biasing potential and its gradients from grids.
colvars: successfully read 0 explicit hills from state.
colvars: Restarted metadynamics bias "mtd" with step number 0.
colvars: Metadynamics bias "mtd": reading the state of replica "3" from file "walk_3/md.colvars.mtd.3.state".
colvars: successfully read the biasing potential and its gradients from grids.
colvars: successfully read 0 explicit hills from state.
colvars: Restarted metadynamics bias "mtd" with step number 0.
colvars: Metadynamics bias "mtd": reading the state of replica "4" from file "walk_4/md.colvars.mtd.4.state".
colvars: successfully read the biasing potential and its gradients from grids.
colvars: successfully read 0 explicit hills from state.
colvars: Restarted metadynamics bias "mtd" with step number 0.
colvars: Metadynamics bias "mtd": reading the state of replica "5" from file "walk_5/md.colvars.mtd.5.state".
colvars: successfully read the biasing potential and its gradients from grids.
colvars: successfully read 0 explicit hills from state.
colvars: Restarted metadynamics bias "mtd" with step number 0.
colvars: Metadynamics bias "mtd": reading the state of replica "6" from file "walk_6/md.colvars.mtd.6.state".
colvars: successfully read the biasing potential and its gradients from grids.
colvars: successfully read 0 explicit hills from state.
colvars: Restarted metadynamics bias "mtd" with step number 0.
colvars: Metadynamics bias "mtd": reading the state of replica "7" from file "walk_7/md.colvars.mtd.7.state".
colvars: successfully read the biasing potential and its gradients from grids.
colvars: successfully read 0 explicit hills from state.
colvars: Restarted metadynamics bias "mtd" with step number 0.
colvars: Synchronizing (emptying the buffer of) trajectory file "md.colvars.traj".

To make start of the simulation be possible I was in need to create for every walker md.colvars.mtd.replicaID.state myself, otherwise it was crashing immediately. I just copied file created in replica 0 to other directories, renamed them and change inside the replica ID to the appropriate number.

As I said, problem is somewhere in the hill deposition, I suppose, since running GDB indicates that problem is somewhere in colvarbias_meta::update_bias() [clone .cold]

Moreover, I run the same simulation without multiple walkers (just erased those lines from metadynamics and didn't change anything else) and it was running well and deposing hills just fine.

My colvars setup file looks like that:

indexFile index.ndx
colvarsRestartFrequency 1000000
colvarsTrajFrequency 100
colvar {
name angle
width 2.0
lowerBoundary -180.0
upperBoundary 180.0
spinAngle {
atoms { indexGroup strand }
refPositionsFile all.xyz
axis (1.0, 0.0, 0.0)
}
}

colvar {
name dist
lowerBoundary 1.4
upperBoundary 5.2

width 0.1
distanceZ {
main {indexGroup Quat }
ref {dummyAtom (0, 0, 0)}
axis (1, 0, 0)
}

}

###################RESTRAINS######################

colvar {
name rmsd
rmsd {
atoms {
indexGroup tail
rotateToReference off
}
refPositionsFile all.xyz
}

}

harmonicWalls {
name wall_max
colvars dist
upperWalls 4.6
upperWallConstant 1000.0
#stepZeroData on
}

harmonicWalls {
name wall_min
colvars dist
lowerWalls 2.0
lowerWallConstant 1000.0

}
harmonic {
colvars rmsd
centers 0.0
forceConstant 10000.0
outputCenters on

}
######################################################

metadynamics {
name mtd
useGrids on
colvars angle dist
newHillFrequency 2000
hillWeight 0.5
hillWidth 1.0
keepFreeEnergyFiles on
writeHillsTrajectory on
wellTempered on
biasTemperature 2790
outputFreq 5000000
multipleReplicas on
replicasRegistry replicasRegistry.txt
replicaUpdateFrequency 10000
replicaID 0
}

I hope it will be possible to find the source of the issue.
Thank you in advance