A computer-implemented method of characterizing chemical composition of a sample containing crude oil or a petroleum fraction, the method comprising: in a processor: (i) receiving assay data comprising molecular-level assay data of the sample, or molecular-level assay data and traditional assay data of the sample, wherein the traditional assay data comprises one or more of distillation curve data, density curve data, sulfur curve data, basic nitrogen curve data, total nitrogen curve data, carbon-to-hydrogen ratio curve data, total acid number curve data, PIONA (paraffins, isoparaffins, olefins, naphthenes and aromatics) content curve data, viscosity curve data, nickel content curve data, and vanadium content curve data; (ii) setting a) absolute compound compositions based on the molecular-level assay data of the sample, b) first compound classes and class weight(s), first conceptual segment type(s), and first segment distribution based on at least part of the molecular-level assay data, and c) second compound classes and class weight(s), second conceptual segment type(s), and second segment distribution based on at least part of the traditional assay data of the sample, if traditional assay data is received; (iii) determining a) absolute compound compositions as the set absolute compound compositions, b) first compound composition(s) from first segment distribution values, which represent the first segment distribution, and first class weight(s) set based on at least part of the molecular level data, and c) second compound composition(s) from second segment distribution parameters, which represent the second segment distribution, and second class weight(s) set based on the traditional assay data of the sample, if traditional assay data is received; (iv) reconciling a) the absolute compound compositions, b) the first compound composition(s), and/or c) second compound composition(s), thereby obtaining a reconciled compound composition; and (v) adjusting, when the molecular-level assay data includes qualitative molecular level assay data, first class weight(s) and/or when the received assay data includes traditional assay data, second class weight(s) and second segment distribution parameters, until physical and/or chemical properties determined for the reconciled compound composition are consistent with corresponding received assay data, thereby obtaining a refined compound composition as the characterization of the chemical composition of the sample. (i) receiving assay data comprising molecular-level assay data of the sample, or molecular-level assay data and traditional assay data of the sample, wherein the traditional assay data comprises one or more of distillation curve data, density curve data, sulfur curve data, basic nitrogen curve data, total nitrogen curve data, carbon-to-hydrogen ratio curve data, total acid number curve data, PIONA (paraffins, isoparaffins, olefins, naphthenes and aromatics) content curve data, viscosity curve data, nickel content curve data, and vanadium content curve data; (ii) setting a) absolute compound compositions based on the molecular-level assay data of the sample, b) first compound classes and class weight(s), first conceptual segment type(s), and first segment distribution based on at least part of the molecular-level assay data, and c) second compound classes and class weight(s), second conceptual segment type(s), and second segment distribution based on at least part of the traditional assay data of the sample, if traditional assay data is received; (iii) determining a) absolute compound compositions as the set absolute compound compositions, b) first compound composition(s) from first segment distribution values, which represent the first segment distribution, and first class weight(s) set based on at least part of the molecular level data, and c) second compound composition(s) from second segment distribution parameters, which represent the second segment distribution, and second class weight(s) set based on the traditional assay data of the sample, if traditional assay data is received; (iv) reconciling a) the absolute compound compositions, b) the first compound composition(s), and/or c) second compound composition(s), thereby obtaining a reconciled compound composition; and (v) adjusting, when the molecular-level assay data includes qualitative molecular level assay data, first class weight(s) and/or when the received assay data includes traditional assay data, second class weight(s) and second segment distribution parameters, until physical and/or chemical properties determined for the reconciled compound composition are consistent with corresponding received assay data, thereby obtaining a refined compound composition as the characterization of the chemical composition of the sample.