The paper by K. M. Ho, C. T. Chan, and C. M. Soukoulis investigates the photonic band structure in periodic dielectric structures using a plane-wave expansion method to solve Maxwell's equations. They find that face-centered-cubic (fcc) dielectric structures do not exhibit a full photonic band gap, despite experimental observations suggesting otherwise. However, they discover that diamond structures of dielectric spheres do possess a full photonic band gap, even at low refractive-index contrasts as low as 2. The authors attribute the absence of a full gap in fcc structures to symmetry degeneracies at specific points in the Brillouin zone, which prevent the formation of a gap. In contrast, the diamond structure's symmetry allows for the formation of a gap, making it a promising candidate for practical applications in photonic devices. The study highlights the importance of considering the vector nature of electromagnetic fields in theoretical calculations, as the scalar-wave approximation often leads to incorrect results.The paper by K. M. Ho, C. T. Chan, and C. M. Soukoulis investigates the photonic band structure in periodic dielectric structures using a plane-wave expansion method to solve Maxwell's equations. They find that face-centered-cubic (fcc) dielectric structures do not exhibit a full photonic band gap, despite experimental observations suggesting otherwise. However, they discover that diamond structures of dielectric spheres do possess a full photonic band gap, even at low refractive-index contrasts as low as 2. The authors attribute the absence of a full gap in fcc structures to symmetry degeneracies at specific points in the Brillouin zone, which prevent the formation of a gap. In contrast, the diamond structure's symmetry allows for the formation of a gap, making it a promising candidate for practical applications in photonic devices. The study highlights the importance of considering the vector nature of electromagnetic fields in theoretical calculations, as the scalar-wave approximation often leads to incorrect results.