We have used highly accurate quantum Monte Carlo methods to determine the chemical structure and electronic band gaps of monolayer GeSe. Two-dimensional (2D) monolayer GeSe has received a great deal of attention due to its unique thermoelectric, electronic, and optoelectronic properties with a wide range of potential applications. Density functional theory (DFT) methods have usually been applied to obtain optical and structural properties of bulk and 2D GeSe. For the monolayer, DFT typically yields a larger band-gap energy than for bulk GeSe but cannot conclusively determine if the monolayer has a direct or indirect gap. Moreover, the DFT-optimized lattice parameters and atomic coordinates for monolayer GeSe depend strongly on the choice of approximation for the exchange-correlation functional, which makes the ideal structure—and its electronic properties—unclear. In order to obtain accurate lattice parameters and atomic coordinates for the monolayer, we use a surrogate Hessian-based parallel line search within diffusion Monte Carlo to fully optimize the GeSe monolayer structure. The DMC-optimized structure is different from those obtained using DFT, as are calculated band gaps. The potential energy surface has a shallow minimum at the optimal structure. This, combined with the sensitivity of the electronic structure to strain, suggests that the optical properties of monolayer GeSe are highly tunable by strain.

中文翻译：

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量子蒙特卡洛法优化单层GeSe的结构和电子带隙

我们已经使用了高精度的量子蒙特卡洛方法来确定单层GeSe的化学结构和电子带隙。二维（2D）单层GeSe由于其独特的热电，电子和光电特性以及广泛的潜在应用而备受关注。密度泛函理论（DFT）方法通常已用于获得块体和2D GeSe的光学和结构性质。对于单层，DFT通常会比块状GeSe产生更大的带隙能量，但无法最终确定单层是具有直接间隙还是间接间隙。此外，单层GeSe的DFT优化晶格参数和原子坐标在很大程度上取决于交换相关函数的近似选择，这使得理想的结构及其电子特性不清楚。为了获得单层的准确晶格参数和原子坐标，我们在扩散蒙特卡洛中使用基于Hessian的替代并行线搜索来完全优化GeSe单层结构。DMC优化的结构与使用DFT获得的结构不同，计算出的带隙也不同。在最佳结构下，势能面的最小值很小。结合电子结构对应变的敏感性，这表明单层GeSe的光学特性可通过应变进行高度调整。我们在扩散蒙特卡洛中使用基于Hessian的替代平行线搜索来完全优化GeSe单层结构。DMC优化的结构与使用DFT获得的结构不同，计算出的带隙也不同。在最佳结构下，势能面的最小值很小。结合电子结构对应变的敏感性，这表明单层GeSe的光学特性可通过应变进行高度调整。我们在扩散蒙特卡洛中使用基于Hessian的替代平行线搜索来完全优化GeSe单层结构。DMC优化的结构与使用DFT获得的结构不同，计算出的带隙也不同。在最佳结构下，势能面的最小值很小。结合电子结构对应变的敏感性，这表明单层GeSe的光学特性可通过应变进行高度调整。