Molecular simulation is of growing importance in chemical engineering. The gap usually present in knowledge representations between disciplines is, however, a serious obstacle for efficient utilization of knowledge. The way of bridging the gap between macroscopic and microscopic characterization of matter presented in this paper is based on the view that the solution for integrating knowledge and for enhancing communication, in general, is possible by developing and adopting a unified concept system (PSSP language). The advantage of this system is that the same concepts can be used both in macroscopic and microscopic worlds, whereby the presentational gap is eliminated. Knowledge integration by the formal approach is illustrated by an example starting from an informal description of the example process which is represented as an object with its purpose and some features of its structure and state as specified. The structure of the process is then detailed hierarchically down to the molecular level. It is emphasized that structural detailing should be done in a manner which supports the subsequent state aggregation. The PSSP language can not eliminate discontinuities, which are in knowledge itself but precise location of the knowledge gap and its nature become more detectable by this kind of formal presentation.