Hi Alicia,
Sorry, I though that the configuration information that you had pasted in your email was only part of your entire configuration file, not the whole thing. You do need more sections that specify things like the substitution model that you want to use. I also did not previously notice that you hadn't specified an alignment file on the datafname line. I pasted in the default configuration file below, with a few changes to set it to do what you want. Note that many of the settings relate to treesearch details, and have no effect when optimizeinputonly is set. You'll need to specify your alignment file as datafname, and your treefile will need to match the name specified as streefname (TreeNex below). The default substitution model is set to GTR+I+G (the most complicated model), but could be changed by making edits in the [model1] section.
One final note, from what you've said I've been assuming that what you want to do is to find the optimal branch lengths (and model parameters) under the ML criterion for the tree returned by TNT. If I've misinterpreted that, let me know.
Best,
Derrick
[general]
datafname = your_alignment_filename_here
constraintfile = none
streefname = TreeNex
attachmentspertaxon = 50
ofprefix = TreeNex.out
randseed = -1
availablememory = 512
logevery = 10
saveevery = 100
refinestart = 1
outputeachbettertopology = 0
outputcurrentbesttopology = 0
enforcetermconditions = 1
genthreshfortopoterm = 20000
scorethreshforterm = 0.05
significanttopochange = 0.01
outputphyliptree = 0
outputmostlyuselessfiles = 0
writecheckpoints = 0
restart = 0
outgroup = 1
resampleproportion = 1.0
inferinternalstateprobs = 0
outputsitelikelihoods = 0
optimizeinputonly = 1
collapsebranches = 1
searchreps = 2
bootstrapreps = 0
[model1]
datatype = nucleotide
ratematrix = 6rate
statefrequencies = estimate
ratehetmodel = gamma
numratecats = 4
invariantsites = estimate
[master]
nindivs = 4
holdover = 1
selectionintensity = 0.5
holdoverpenalty = 0
stopgen = 5000000
stoptime = 5000000
startoptprec = 0.5
minoptprec = 0.01
numberofprecreductions = 10
treerejectionthreshold = 50.0
topoweight = 1.0
modweight = 0.05
brlenweight = 0.2
randnniweight = 0.1
randsprweight = 0.3
limsprweight = 0.6
intervallength = 100
intervalstostore = 5
limsprrange = 6
meanbrlenmuts = 5
gammashapebrlen = 1000
gammashapemodel = 1000
uniqueswapbias = 0.1
distanceswapbias = 1.0