Pygame Wave Equation

import pygame
import math
import random
from pygame.locals import *

#setup

background_colour = (0,0,0)
(width, height) = (500, 500)

screen = pygame.display.set_mode((width, height))#,pygame.FULLSCREEN)
screen.fill(background_colour)
pygame.display.set_caption('Wave equation')
pygame.font.init()

myfont = pygame.font.SysFont("monospace", 20)

clock = pygame.time.Clock()

class particle:
	def __init__(self, i, j):
		self.x = i
		self.y = j
		self.value = 0
		self.velocity = 0
		self.neighbours = []

	def get_neighbours(self):
		if self.x >= 1 and self.x <= size_x - 2 and self.y >= 1 and self.y <= size_y - 2:
			self.neighbours.append(values[self.x - 1][self.y])
			self.neighbours.append(values[self.x + 1][self.y])
			self.neighbours.append(values[self.x][self.y - 1])
			self.neighbours.append(values[self.x][self.y + 1])

	def update(self):
		if self.x >= 1 and self.x <= size_x - 2 and self.y >= 1 and self.y <= size_y - 2:
			lap = 0
			for n in self.neighbours:
				lap += n.value
			lap -= 4*self.value
			self.velocity += 0.01*lap
			self.velocity *= 0.97
			self.value += self.velocity
		else:
			self.value = 0

size_x = 50
size_y = 50

values = []
for i in range(size_x):
	values.append([])
	for j in range(size_y):
		values[i].append(particle(i,j))

for i in range(size_x - 1):
	for j in range(size_y - 1):
		values[i][j].get_neighbours()

#values[int(size_x/2)][int(size_y/2)].value = 20


running = True
while running:
	for event in pygame.event.get():
	    if event.type == pygame.QUIT:
	        running = False
	    elif event.type == KEYDOWN:
	        if event.key == K_ESCAPE:
	            running = False


	screen.fill(background_colour)

	pos = pygame.mouse.get_pos()
	grid_pos = [int(pos[0]/10), int(pos[1]/10)]
	values[grid_pos[0]][grid_pos[1]].value = 20

	for i in range(size_x):
		for j in range(size_x):
			values[i][j].update()
			pygame.draw.circle(screen,
				[30,min(255,max(0,100 - 10*values[i][j].value)),min(255,max(0,100 + 10*values[i][j].value))],
				[10*i,10*j], 7)

	pygame.display.flip()

	clock.tick(60)



pygame.quit()