The vp_setup.dat file should contain LASTCHTIED v -- or whatever letter you choose. Later letters are then treated in a special way for tieing column densities so that the first one in the block gives the TOTAL column density for the complex. The redshift and Doppler parameter still apply to that individual component, which now has a column density equal to the list value minus the sum of all the rest with the same identifying letter. Thus order in the fort.13 file is important!

For example, if you want to know the total column density on the assumption that an HI system is made up of b=13 clouds, then you would put in fort.13 something like:

H I 1.3905E+01 3.320368 9.65 0.00 0.00E+00 0
H I 1.6766E+01x 3.320970 13.00F 0.00 1.00E+00 0
H I 1.5914E+01x 3.321462 13.00G 0.00 1.00E+00 0

The 1.6766E+01 is then the sum of the column densities in the two 'x' systems. This may seem pointless, but in fact is useful for close blends in which the error estimates for the individual components are large because of difficulties in resolving the blend, while the total column density is much better constrained.

Now to tie different ions. You can do this by having fort.13 containing e.g.:

C IV 1.4632E+01x 2.765821a 12.69i 0.00 1.00E+00 0
C IV 1.3894E+01x 2.765965b 17.73j 0.00 1.00E+00 0
SiIV 1.3968E+01% 2.765821A 12.69I 0.00 1.00E+00 0
SiIV 1.3101E+01X 2.765965B 17.73J 0.00 1.00E+00 0

This will result in the first lines for each ion containing TOTAL column densities for that ion in the complex, and, in this example, the SiIV and CIV are constrained to have the same ratio in each velocity component. Note that for your first guesses you don't have to get the ratios right -- the program sorts it out by making all ratios match the first before starting to iterate. The most hideous example of this, which has been used in anger, is the following:

SiII 1.4947E+01x 3.390694a 21.56i 0.00 0.00E+00 0
SiII 1.4586E+01x 3.390543b 4.85j 0.00 0.00E+00 0
SiII 1.2426E+01x 3.388740c 18.02k 0.00 0.00E+00 0
SiII 1.3483E+01x 3.390069d 6.83l 0.00 0.00E+00 0
SiII 1.4072E+01x 3.390359e 1.46m 0.00 0.00E+00 0
SiII 1.4462E+01x 3.390715f 3.89n 0.00 0.00E+00 0
SiII 1.3057E+01x 3.391030g 7.45o 0.00 0.00E+00 0
SiII 1.3689E+01x 3.391562h 0.58p 0.00 0.00E+00 0
O I 1.5890E+01% 3.390694A 28.57I 0.00 0.00E+00 0
O I 1.5528E+01X 3.390543B 6.42J 0.00 0.00E+00 0
O I 1.3369E+01X 3.388740C 23.87K 0.00 0.00E+00 0
O I 1.4426E+01X 3.390069D 9.05L 0.00 0.00E+00 0
O I 1.5014E+01X 3.390359E 1.94M 0.00 0.00E+00 0
O I 1.5404E+01X 3.390715F 5.16N 0.00 0.00E+00 0
O I 1.3999E+01X 3.391030G 9.86O 0.00 0.00E+00 0
O I 1.4631E+01X 3.391562H 0.77P 0.00 0.00E+00 0
FeII 1.4977E+01% 3.390694A 15.29I 0.00 0.00E+00 0
FeII 1.4615E+01X 3.390543B 3.44J 0.00 0.00E+00 0
FeII 1.2456E+01X 3.388740C 12.78K 0.00 0.00E+00 0
FeII 1.3513E+01X 3.390069D 4.85L 0.00 0.00E+00 0
FeII 1.4101E+01X 3.390359E 1.04M 0.00 0.00E+00 0
FeII 1.4491E+01X 3.390715F 2.76N 0.00 0.00E+00 0
FeII 1.3086E+01X 3.391030G 5.28O 0.00 0.00E+00 0
FeII 1.3718E+01X 3.391562H 0.41P 0.00 0.00E+00 0
AlII 1.3670E+01% 3.390694A 22.00I 0.00 0.00E+00 0
AlII 1.3308E+01X 3.390543B 4.94J 0.00 0.00E+00 0
AlII 1.1149E+01X 3.388740C 18.38K 0.00 0.00E+00 0
AlII 1.2206E+01X 3.390069D 6.97L 0.00 0.00E+00 0
AlII 1.2794E+01X 3.390359E 1.49M 0.00 0.00E+00 0
AlII 1.3184E+01X 3.390715F 3.97N 0.00 0.00E+00 0
AlII 1.1779E+01X 3.391030G 7.60O 0.00 0.00E+00 0
AlII 1.2411E+01X 3.391562H 0.59P 0.00 0.00E+00 0
H I 1.3719E+01 3.703667 112.68 0.00 0.00E+00 0
H I 1.3428E+01 3.705300 38.23 0.00 0.00E+00 0
H I 1.3309E+01 3.706932 14.70 0.00 0.00E+00 0
H I 1.2646E+01 3.710021 13.12 0.00 0.00E+00 0
H I 1.4274E+01 3.711302 6.22 0.00 0.00E+00 0
H I 1.4098E+01 3.711801 6.47 0.00 0.00E+00 0
H I 1.3884E+01 3.712458 17.23 0.00 0.00E+00 0
H I 1.3939E+01 4.092741 28.55 0.00 0.00E+00 0
H I 1.2524E+01 4.094299 14.34 0.00 0.00E+00 0
H I 1.3384E+01 4.095774 51.80 0.00 0.00E+00 0
H I 1.3735E+01 4.097573 22.66 0.00 0.00E+00 0
H I 1.3006E+01 4.100405 11.91 0.00 0.00E+00 0
H I 1.4224E+01 4.101517 28.68 0.00 0.00E+00 0
H I 1.6541E+01 4.101878 8.46 0.00 0.00E+00 0
H I 1.3005E+01 4.107944 40.82 0.00 0.00E+00 0
H I 1.3394E+01 4.109549 35.37 0.00 0.00E+00 0
H I 1.4156E+01 3.298595 119.78 0.00 0.00E+00 0
H I 1.4616E+01 3.303102 49.14 0.00 0.00E+00 0
H I 1.4130E+01 3.299795 53.09 0.00 0.00E+00 0
H I 1.3280E+01 3.308690 15.30 0.00 0.00E+00 0
H I 1.3698E+01 3.310201 59.30 0.00 0.00E+00 0
H I 1.2483E+01 3.306753 14.20 0.00 0.00E+00 0
C IV 1.3237E+01v 3.728854 19.77 0.00 0.00E+00 0
C IV 1.2603E+01v 3.730043 1.27 0.00 0.00E+00 0
H I 1.3680E+01 3.301629 18.18 0.00 0.00E+00 0

In this case the SiII values are treated as the base block, and the first of them is the total. The OI, FeII and AlII values are tied to have the same relative numbers between redshift components as the SiII, with the totals for each free to vary (the ones marked %). Other lines (such as HI) are included to get the fit right.

This highlights another minor point -- for complex systems you might run out of letters in the alphabet! No worries, just use two letters. The first is the one checked for case, the second is just there as a case-independent marker. So aa and ab are distinct base values, with subsequent values marked by AA (or Aa) and AB respectively tied to these base values. So you could have, with equivalent effect to above:

SiII 1.4947E+01x 3.390694aa 21.56ai 0.00 0.00E+00 0
SiII 1.4586E+01x 3.390543ab 4.85aj 0.00 0.00E+00 0
SiII 1.2426E+01x 3.388740c 18.02k 0.00 0.00E+00 0
SiII 1.3483E+01x 3.390069d 6.83l 0.00 0.00E+00 0
SiII 1.4072E+01x 3.390359e 1.46m 0.00 0.00E+00 0
SiII 1.4462E+01x 3.390715f 3.89n 0.00 0.00E+00 0
SiII 1.3057E+01x 3.391030g 7.45o 0.00 0.00E+00 0
SiII 1.3689E+01x 3.391562h 0.58p 0.00 0.00E+00 0
O I 1.5890E+01% 3.390694AA 28.57AI 0.00 0.00E+00 0
O I 1.5528E+01X 3.390543AB 6.42AJ 0.00 0.00E+00 0
O I 1.3369E+01X 3.388740C 23.87K 0.00 0.00E+00 0
O I 1.4426E+01X 3.390069D 9.05L 0.00 0.00E+00 0
O I 1.5014E+01X 3.390359E 1.94M 0.00 0.00E+00 0
O I 1.5404E+01X 3.390715F 5.16N 0.00 0.00E+00 0
O I 1.3999E+01X 3.391030G 9.86O 0.00 0.00E+00 0
O I 1.4631E+01X 3.391562H 0.77P 0.00 0.00E+00 0
FeII 1.4977E+01% 3.390694AA 15.29AI 0.00 0.00E+00 0
.. etc

Note forcing the ratios to be the same will affect the error estimates .. things are no longer independent. The errors flagged with % are those in the total column density given the ratios between components as error-free (I think).