## 1 Introduction
The last two formats mentioned are the most widely used open standards for such data exchange. IFC allows transferring complex data about a building project, but it is rather complicated and creates large files, while gbXML offers a more flexible and direct approach to energy analysis. Both standards transfer information about materials, HVAC (heating, ventilation, and air conditioning) systems, and thermal zones, but only gbXML also contains location data [32]. The communication between BIM and BEM depends on data exchange files, which are instrumental for information transfer. Both formats, IFC and gbXML, are supported by most software tools for energetic simulations [33]. Specifically, the IFC is an ISO standard (International Organization for Standardization) covering a large class of domains of civil engineering [34; 35], supported by the buildingSMART platform [36]. [PERSON] [37] suggested the application of IFC for coupling the software tools for building design with tools oriented on energy evaluations (since 2002). The upgraded versions of IFC include numerous BIM standards, namely IFC4 (in use since 2013) includes approximately 800 entities, 258 attribute sets, and 121 data types. However, apart from good possibilities for analysing the thermal performance of buildings, numerous relevant software tools still have difficulties with data import and utilisation of the IFC. A concurrent format, gbXML, was developed in 2002, thanks to the financial support of the U.S. Department of Energy and the companies Autodesk, ASHRAE, and BentleySystems, as an open standard for BIM with good abilities to share information about buildings. This format can characterise all attributes of buildings, thanks to its 346 elements and 167 simple types; this is appreciated especially by experts in the area of environmental demands of buildings [38]. In the last several years, due to the official statistics of organisations managing standards connected with IFC and gbXML formats, 45 software packages support data transfer based on IFC, whereas only 15 packages support that based on gbXML. The mechanisms and forms of both IFC and gbXML descriptions of relevant information on environmental demands of buildings were analysed by [PERSON] et al. ; the mapping of entities between both formats [39] forms a base for mutual data conversion. Numerous software applications for thermal numerical simulations, such as Green Building Studio, PKPM-Energy, HY-EP, DesignBuilder, or OpenStudio, rely exclusively on the gbXML format for data import. Nevertheless, the interaction process has many disadvantages, difficulties, and errors. For example, an exported file from the Autodesk Revit platform contains only information about the building geometry, whereas information on the thermal technical characteristics of specific building envelope layers is missing. Using the choice Rooms/Spaces, the conversion is more appropriate but the data on HVAC systems are still not available [3; 40]. The mechanism of conversion from BIM to BEM has been discussed by many authors. More specifically, [PERSON] et al. [41], using their analysis of existing methods of conversion from BIM to BEM, developed a method for transferring BIM models to 2D graphics, followed by an automatic reconstruction of information from BIM. Unfortunately, all geometrical calculations are performed in 2D; therefore, roofs, outer shading, and various special structures are not effectively included. [PERSON] [42] tries to improve this to simulate energy consumption in green buildings, utilising selected advantages of the IFC format. The complex evaluation of the process of data import and the setting of software parameters for energy simulations in the above-introduced six software tools has been performed by [PERSON] et al. [43], yet further improvements are still required [31]. As emphasised by [PERSON] et al. [29], both BEM and BIM rely on replaceable formats IFC and gbXML, which are able to store graphic information together with certain attributed data.