Final answer:
X-ray diffraction is considered the most accurate method to calculate nanocrystal sizes due to its ability to measure the deflection of X-rays, with histological data supporting the size-dependent behavior of nanoparticles in biological systems.
Step-by-step explanation:
Different models, such as X-ray analysis, and considering molecular weight for single molecules, provide us with various ways to calculate the size of nanocrystals. However, X-ray diffraction appears to be the most scientifically reliable method for obtaining accurate sizes of atoms or ions as it allows for the calculation of size by measuring the deflection of X-rays through a crystal. The size of nanocrystals can be critical in applications such as drug delivery where smaller particles have been shown to have better tissue penetration, as indicated by histological data from tumor studies.
Other modeling methods, while useful in their respective contexts, may not provide the same level of precise measurement, especially when applied to complex structures. The understanding and measurement of atomic and nanoparticle sizes continue to evolve with developments in techniques and technologies.