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I know VASP can do it. But how?
Thank you.

you have to calculate a partial charge density. First you do a static selfconsistent calculation(you need the WAVECAR) of your system and then you can do an extra calculation setting the tag LPARD=.TRUE. in the incar file.
NBMOD sets how you will be calculating your partial charge density (eg NBMOD=-3 sets it such that you define the energy interval)
EINT sets the energy interval. If you put only 1 value there the partial charge is calculated from that value to the fermi level, if you put 2 values then they will be the boundaries of the energyinterval.

So if you want to make a speudo-STM with a sample bias of -1.50V then you can use
LPARD=.TRUE.
NBMOD=-3
EINT=-1.5
more info can be found in the manual:
http://cms.mpi.univie.ac.at/vasp/vasp/node154.html
(slightly hidden, probably to make sure you read the entire manual, not just the part wich gives the parameters )

After this calculation you get a file PARCHG which is setup exactly like a CHGCAR file and from that point on making the STM image is peanuts. Just write a nice little program which goes trough the PARCHG file and generates the isosurface you are looking for.

There are a few detail you need to keep in mind:
The WAVECAR you generated in the static calculation needs to be read correctly by vasp...any excuse it gets it will use to generate a new random WAVECAR file. So best is to set also NBANDS in your INCAR file to the same amount you had in your static calculation...and if you are running on multiple cpu's make sure that NBANDS is a multiple of the number of CPU's otherwise VASP will ignore NBANDS and still kill your result...(and as no errorchecking/handling is done by VASP you will have to make sure of this yourself, the job won't crash it will just give you wrong results)

Danny

nice reply

thanks for sharing the post