A method of determining characteristics of predetermined molecules using a processor, the method comprising: the use of a combined ion mobility-mass spectrometry (IM-MS) technique for experimentally determining a range Z of molecular structures; estimating the cross sectional area of a molecule comprising the steps of: A) Predicting the geometric structure of the molecule of interest; B) Assigning a value for the cross sectional area of each atom within the molecule; C) Calculating the sum of the values of the cross sectional area for each atom within the molecule; D) Randomly selecting an orientation of the predicted geometric structure of said molecule of interest; E) Randomly selecting one or more atoms with probability proportional to the atom's cross sectional area; F) Randomly selecting a position within the cross sectional area of the or each of the selected atom(s); G) Identifying the number of atoms whose cross sectional area includes said randomly selected position; H) Calculating a value for the cross sectional area of said molecule of interest at said randomly selected orientation; I) Calculating an averaged cross sectional area over all said randomly selected orientations; J) Iterating steps D) to I) for at least N iterations and/or until M independent calculations of the averaged cross sectional area agree within a predetermined range R for X iterations; and comparing values of said range Z with those estimated, wherein the method correlates the differences in the molecular structures with differences in selected predetermined activity of those molecules. the use of a combined ion mobility-mass spectrometry (IM-MS) technique for experimentally determining a range Z of molecular structures; estimating the cross sectional area of a molecule comprising the steps of: A) Predicting the geometric structure of the molecule of interest; B) Assigning a value for the cross sectional area of each atom within the molecule; C) Calculating the sum of the values of the cross sectional area for each atom within the molecule; D) Randomly selecting an orientation of the predicted geometric structure of said molecule of interest; E) Randomly selecting one or more atoms with probability proportional to the atom's cross sectional area; F) Randomly selecting a position within the cross sectional area of the or each of the selected atom(s); G) Identifying the number of atoms whose cross sectional area includes said randomly selected position; H) Calculating a value for the cross sectional area of said molecule of interest at said randomly selected orientation; I) Calculating an averaged cross sectional area over all said randomly selected orientations; J) Iterating steps D) to I) for at least N iterations and/or until M independent calculations of the averaged cross sectional area agree within a predetermined range R for X iterations; and A) Predicting the geometric structure of the molecule of interest; B) Assigning a value for the cross sectional area of each atom within the molecule; C) Calculating the sum of the values of the cross sectional area for each atom within the molecule; D) Randomly selecting an orientation of the predicted geometric structure of said molecule of interest; E) Randomly selecting one or more atoms with probability proportional to the atom's cross sectional area; F) Randomly selecting a position within the cross sectional area of the or each of the selected atom(s); G) Identifying the number of atoms whose cross sectional area includes said randomly selected position; H) Calculating a value for the cross sectional area of said molecule of interest at said randomly selected orientation; I) Calculating an averaged cross sectional area over all said randomly selected orientations; J) Iterating steps D) to I) for at least N iterations and/or until M independent calculations of the averaged cross sectional area agree within a predetermined range R for X iterations; and comparing values of said range Z with those estimated, wherein the method correlates the differences in the molecular structures with differences in selected predetermined activity of those molecules.