In the present paper, we give the formalism for further developing a fully three-dimensional HZETRN code using marching procedures but also development of a new Green's function code is discussed. The final Green's function code is capable of not only validation in the space environment but also in ground based laboratories with directed beams of ions of specific energy and characterized with detailed diagnostic particle spectrometer devices. Special emphasis is given to verification of the computational procedures and validation of the resultant computational model using laboratory and spaceflight measurements. Due to historical requirements, two parallel development paths for computational model implementation using marching procedures and Green s function techniques are followed. A new version of the HZETRN code capable of simulating HZE ions with either laboratory or space boundary conditions is under development. Validation of computational models at this time is particularly important for President Bush s Initiative to develop infrastructure for human exploration with first target demonstration of the Crew Exploration Vehicle (CEV) in low Earth orbit in 2008.Wilson, John W. and Tripathi, Ram K. and Mertens, Christopher J. and Blattnig, Steve R. and Clowdsley, Martha S. and Cucinotta, Francis A. and Tweed, John and Heinbockel, John H. and Walker, Steven A. and Nealy, John E.Johnson Space Center; Langley Research CenterFORMALISM; MATHEMATICAL MODELS; PROGRAM VERIFICATION (COMPUTERS); COMPUTER PROGRAMS; SPACECRAFT DESIGN; BOUNDARY CONDITIONS; SIMULATION; GREEN'S FUNCTIONS; RADIATION SHIELDING; LOW EARTH ORBITS; AEROSPACE ENVIRONMENTS; CREW EXPLORATION VEHICLE; INTERNATIONAL SPACE STATION; LINEAR ENERGY TRANSFER (LET); EXTRATERRESTRIAL RADIATION...