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CASTEP STM profiles in Materials Studio

I am investigating STM profiles in some defected graphene structures however I am confused with the description in the manual.
It says, 

"CASTEP models the STM profile by representing it as an isosurface of the electron density generated only by states at a certain energy away from the Fermi level. The distance from the Fermi level corresponds to the applied bias in STM experiments: positive bias corresponds to empty (conduction) states and negative bias to occupied (valence) states. This approach neglects the actual geometry of the STM tip" 

Does this mean the isosurfaces generated by this correspond to all the states up to the applied voltage (for a positive bias) above the Fermi level?  

If it does not, how can I generate the above similar to the LDOS. (Integration of the density of states at the vicinity of the fermi level)

Also how does the applied bias relate to the energy of the density of states?
I need to calculate exactly up to which energy in the density of states, an applied bias (let's say 1V) represents. 
Furthermore, what are the units of the isosurface created by the STM profile? 

I would also appreciate if you could direct me to any reading regarding this as I could not find any relevant information in the manual. 

Thank You,
- Asanka