THEORSTM is a universal Scanning Tunneling and Atomic Force Microscopy images simulator for interpretation of experimental STM and AFM data of molecule on conductive substrate. Applications include modelling of the system containing the molecule, the support and the STM or AFM tip, calculations of images and current-voltage characteristics, interactive superimposing molecular model into experimental or theoretical image.
THEORSTM makes it possible to predict STM and AFM images for practically complex molecules (up to several hundreds of atoms) is physically adsorbed on the surface of conductive substrate of known structure. In the present versions the program permits to treat adsorbent containing atoms of C,H,O,N,P,Cl elements. As a substrate it may be considered the surface of graphite, 100-surfase of Cu or 100-surfase of Ni.
THEORSTM produces detailed characteristics of energy spectrum, density of states and wave functions of composite system containing the molecule, the support and the tip. If it necessary electronic characteristics of each subsystem of the whole system can be calculated.
Models and Features
The molecule geometry is generated by means of molecular mechanics method.
Three types of model of tip can be considered: three face pyramid, four face pyramid, linear chain of atoms.
The program permits to calculate an electronic configuration of STM object: molecule+conductive substrate by means of two ways. The first way is modified Tersoff-Hamann approximation [see Kurnikov I.V., Sivozhelezov V.S.,Redchenko V.V.,Gritcenko O.V. "An extention of the Tersoff-Hamman model for a molecule on a graphite support: STM image constraction" - Molecular Engeneering, vol.2, p.165-176, 1992]. In this way molecular electronic configuration is calculated by Extended Huckel Method or by CNDO selfconsistend field method. The variation of substrate wave function is taken into account via the perturbation procedure. The second possible way of calculation of electronic structure of STM system: tip+molecule+support is tight binding method with parametrs fitted from first principle calculations for bulk materials. Mixing of molecular and substrate electronic states is performed exactly by means of diagonaligation of composite Hamiltonian of molecule with the substrate. An extraction of variety of electronic characteristics (partial densities of states,local charges) is possible.
Calculation of tunnelling current and Current-Voltage characteristics is based on Bardeen formula which assumes weak interaction of the tip and the molecule-substrate system. Eigen values and eigen vectors of global Hamiltonian are found by Jcoly method.
The phenomenological Lennard-Jones combines the attractive Van-der-Waals potential and repulse atomic potential is used for calculations of AFM image in additive approximation.
Users Interface
The software is menu-driven and easy to use, has an extensive help system. The user can be optionally provided with an interface with any molecular modelling system at the user's convenience. The only input information the THEORSTM package needs is the investigated substance geometry model generated by the molecular modelling system.
Visualisation
Visualisation is performed using the MDT MULTI-IMAGE-MOLPOS software graphics system.
Specification
Personal Computers IBM 386, 486 Compatible
DOS 3.0 or higher
Language FORTRAN, C
Memory Required 16 Mb
Hard Disk Free Space 16 Mb
Required 387 Math Coprocessor
|
![[NT-MDT: Molecular Devices and Tools for NanoTechnology]](label/ntmdt.gif)