Deep Decarbonization Pathways Project researchers have also analyzed model typologies. A 2014 paper outlines the modeling challenges ahead as energy systems become more complex and human and social factors become increasingly relevant. Electricity sector models are used to model electricity systems. The scope may be national or regional, depending on circumstances. For instance, given the presence of national interconnectors, the western European electricity system may be modeled in its entirety. Engineering-based models usually contain a good characterization of the technologies involved, including the high-voltage AC transmission grid where appropriate. Some models (for instance, models for Germany) may assume a single common bus or "copper plate" where the grid is strong. The demand-side in electricity sector models is typically represented by a fixed load profile. Market-based models, in addition, represent the prevailing electricity market, which may include nodal pricing. Game theory and agent-based models are used to capture and study strategic behavior within electricity markets. In addition to the electricity sector, energy system models include the heat, gas, mobility, and other sectors as appropriate. Energy system models are often national in scope, but may be municipal or international. So-called "top-down models" are broadly economic in nature and based on either partial equilibrium or general equilibrium. General equilibrium models represent a specialized activity and require dedicated algorithms. Partial equilibrium models are more common. So-called "bottom-up models" capture the engineering well and often rely on techniques from operations research. Individual plants are characterized by their efficiency curves (also known as input/output relations), nameplate capacities, investment costs (capex), and operating costs (opex). Some models allow for these parameters to depend on external conditions, such as ambient temperature. Producing hybrid top-down/bottom-up models to capture both the economics and the engineering has proved challenging. This section lists some of the major models in use. These are typically run by national governments. In a community effort, a large number of existing energy system models were collected in model fact sheets on the Open Energy Platform. LEAP (Long range Energy Alternatives Planning System) is a software tool for energy policy analysis and climate change mitigation assessment. LEAP was developed at the Stockholm Environment Institute's (SEI) US Center. LEAP can be used to examine city, statewide, national, and regional energy systems. LEAP is normally used for forecast studies of between 20–50 years. Most of its calculations occur at yearly intervals. LEAP allows policy analysts to create and evaluate alternative scenarios and to compare their energy requirements, social costs and benefits, and environmental impacts.